Off-the-beaten-track UK

The North Pennines in general.
There's a Sci Fi museum at Allendale.
Not hit there yet but Blanchland is on my list and the Crewe Arms.
The museum or even staying in the yurts at Killhope Wheel is well worth the stop. Very good chance of seeing red squirrels too
Jordas Cave ,further south in the Dales.
Alston is englands highest town every route there is scenic.

In York there's a book/ map describing a snicket walk through the city
Yew Tree at Cauldon in Staffs is very quirky.
Oh and Druids Circle outside Masham in the Dales
The Morecambe Bay walk

Take a look at the DK Eyewitness Guide "Back Roads Great Britain", which as the title suggests is full of off the beaten path driving adventures highlighting lesser known attractions around the country. We used ours extensively on a 3 week trip to England last year.
You can usually find used copies on Amazon for just a few dollars.

No Man’s Land Fort
Devon’s Gnome Reserve
The Attendant Coffee Roasters in London ( former public toilet)
Clift Lift in Saltburn
Bekcon Scott Model Village and Railway
The Eden Project
Crystal Park Dinosaurs
The Lost Cave in Nottingham
The ossuary in St Leonard’s Church, in Hythe, Kent

One place that never gets a mention on here is > https://www.exmoor-nationalpark.gov.uk Presumably the Americans and Canadians who mainly use this forum have never heard of this scenic area?

https://www.pembrokeshirecoast.wales is another area that is rarely mentioned on this forum. You could travel to this south-western part of Wales via the A40 and visit the Brecon Beacons National Park en-route.

Even in Cumbria- the Lake District- there are lots of little visited villages which the guide books never mention, but quietly reward visitors.
All the tourist guide books concentrate on the small central area of the County.
The Howgills east of Kendal, essentially the area between Kendal and the A66 have lots of places (and lots of hidden history) to visit and lovely walking where you will barely meet a soul all day. Sedbergh (a town spiritually in the Yorkshire Dales, but now in Cumbria) is just a delight to base yourself in.
The East Fellside (the area east of Penrith) is another area full of little villages, wonderful scenery and walks- yet few people venture off the A66.
I've been spending a lot of time in South West Cumbria recently, in the villages off the A590 to Barrow. Take either the back road from Ulverston to Barrow, or the coast road and lots of places to discover. On the inland route every village has it's own big village pond, there's the very historic Church at Urswick and a water mill at Gleaston. Along the coast you come to Conishead Priory and later to Piel Island, with it's little island and the King of Piel. I wrote on another thread recently about the back door routes to the honeypots of Hawkshead and Coniston.
Some of the routes in the Crake Valley (between the A590 and Hawkshead) are so quiet that the roads are gated to protect the livestock, so you keep having to get out of the car to open and shut gates. No distance from the honey spot of Hawkshead you find the little Tottlebank Baptist Chapel in a fold of the hills (a chapel full of history), and there is the Oxen Park Cinema if you are lucky enough to be there when they have a showing- a tiny Cinema in a converted mill you'd never know existed.
Head out west to Millom and area, very easy to get to but overseas visitors never there get. Just stacked with industrial history like the Hodbarrow Mines.
Undoubtedly the best place to stay in that area is Broadgate House with Diana, the Lady Dowager Lewthwaite- a Georgian gem house which has been in the same family for well over 200 years. The Lewthwaite family were one of the great trading families of West Cumbria, with trading interests originally in the Caribbean, but later also in South and East Africa and Malaya. A really historical place. I admit bias here having archivally catalogued a huge part of the family records.
A bit further north, on the edge of Eskdale and Wasdale (two quiet valleys) another stately home you can stay at is Irton Hall.
Only just north of Keswick there is the village of Caldbeck (of John Peel and the Maid of Buttermere fame) with it's church dating back to the 1100's, clog maker, duck pond, old corn mills, bobbin mills and woollen mills, waterfalls on lovely walks and a huge mining history to explore. A lovely country pub and a restaurant in the converted Priests Mill. If you have a car this is a great place to base yourself but very close to both the well known and some of the lesser known Lake District walks.
There's far too much to write about here, and I've only grazed the surface on Cumbria. Cumbria Tourist Board are constantly trying to spread the tourist load outside the central area with mixed success.
Much or all of the above won't even be in RS, as much because no guide book can cover everywhere, and, well, places wouldn't be unknown if they were in the guidebooks!

Almost anywhere along either side of the English/Welsh border is gorgeous and little known. Herefordshire’s Golden Valley is a particular gem.
Also the Ribble Valley and the Forest of Bowland - you will be lucky to find British tourists there, let alone anyone else. It’s a particular gastronomic hub now with 3 of the UK’s best gastropubs (numbers 1,3 and 10) in the area.

The U.K. is not that big and most pretty or interesting areas will get visitors in peak season. However, a lot of places don’t get the huge crowds that descend on the most well known spots.

Northumberland is the least populated county in England so has lots of places that aren’t heavily visited. You won’t find many people from overseas anywhere here, even at Hadrian’s Wall. I love Cragside, a huge National Trust Victorian pile, and the nearby village of Rothbury.

Pembrokeshire is another favourite spot. It’s 3 hours drive from where I live and it feels like another world. So peaceful and beautiful. It gets plenty of visitors but it’s not overrun in peak season like Devon and Cornwall if you stay away from the Tenby area. St David’s does get busy but it’s worth a visit anyway. Fabulous coastal walks.

Everyone who goes to Hadrian's Wall, even with a car, concentrates on Hexham and Haltwhistle, and the best bits of the wall.
No one suggests the other villages of the Tyne Valley such as Bellingham and Otterburn- which both have plenty of small family run places to stay. Yet that area is easily accessible to the wall, and very interesting to explore. Beyond Bellingham is Kielder Water- a protected dark skies area well known to astronomers. Lots of walking of various grades around the reservoir and a ferry on the reservoir to help you or just for a round trip.
And you can drive on north from Kielder into the Scottish Borders to Hawick and the other very interesting Border Towns- another back route between Northern England and Edinburgh which richly rewards those with a car and the urge to explore beyond the guide books.

In Scotland everyone knows of the NC500 driving route in the Highlands. Few people know of the SWC (South West Coastal) 300, through undiscovered parts of Dumfries and Galloway.

The ones that come to me are in Wales: Harold’s Stone in Trelleck, the weeping yew at St. Brynach’s church and Pentre Ifan burial chamber in Nevern, any of the Neolithic sites on Anglesey, Llynon windmill, 6-seat St. Trillo Chapel in Rhos on Sea, Dyfi furnace, Pont Minllyn pack horse bridge, and all the Alice in Wnderland wooden sculptures scattered around Llandudno. One last one is the elf house in Monmouth. It’s across the street from the entrance to Parish Church of Saint Thomas the Martyr.

Stanton Bridge is actually Santon Bridge- the Inn is the home of the Biggest Liar in the World Competition. Just up the hill and round the corner from the Inn is the Woodlands Licensed Bistro and Tea Room- a family favourite place, I was only there a few weeks ago.
Highly recommended if you are ever in the area.
Just a couple of miles away at Netherwasdale/Strands (just off the road to Wasdale so easy to miss it) is the Saw Mill Cafe, in a very sympathetically converted old Saw Mill, which also has a rather good farm shop.
If it restarts this year the park and ride to Wasdale Head itself runs from just opposite the Saw Mill- an attempt to resolve the acute parking problems at the head of the valley.

Not off the track but off the usual tourist beat is the excellent national mining museum up near Wakefield.
Underground tour lasts about an hour plus from memory.
Coldstones Cut overview west of Pateley Bridge.
La Rosa campsite and the associated themed Bnb in Whitby..
The cafe mentioned in Caldbeck is a belter.

Cardingshed also known as the Oil Can Cafe at Holmfirth ...what would Norah Batty say

The church at Hubberholme in the Dales has some work by " the mouseman"
Wharfedale is spectacular The sidedale in the journey over from Malham drops you at Arncliffe home of the Falcon Inn which I still think serves beer via a jug rather than a pump.

One place that never gets a mention on here is > https://www.exmoor-nationalpark.gov.uk Presumably the Americans and Canadians who mainly use this forum have never heard of this scenic area?

One of these days I want to get back to England and visit Exmoor and Dartmoor and Devon and all of the places I first read about in my favorite Agatha Christie murder mysteries. I started reading them in grade school and was fascinated by the areas she described. I've been to England twice but have not yet made it down to the southwest corner, although I'm hoping to in 2024.

I'm definitely bookmarking this thread - thanks for the question, Joanna!

Are you familiar with the Atlas Obscura website? Lots of interesting stuff there. Be sure to poke around within the site to filter by location, expand the listings for more text and pictures, etc.

https://www.atlasobscura.com/things-to-do/united-kingdom

Here's an example of one listing I plan to visit next month.

https://www.atlasobscura.com/places/sir-john-soanes-museum

You have been given some great answers. I'm shocked that no one has mentioned probably the greatest site, and for sure the greatest museum in all of the UK. Located in the town of Keswick in the Lake District is.......

The Derwent Pencil Museum

I can only describe it as....."that's 15 minutes I'll never get back."

YMMV

My mileage varies on the Derwent Pencil Museum! I loved it! The WWII-related things were particularly interesting to me.

IF anyone is an artist, think ahead of time about what pencil sets you want to buy because of course they have the whole Derwent artist quality range in the gift shop. I was so overwhelmed I left with nothing but it was impressive.

https://en.wikipedia.org/wiki/Derwent_Cumberland_Pencil_Company

PS - My visit was before the flood washed it out and well before Covid.

I agree with Pam. The Pencil Museum is a wonderful place.
I guess it might be one of those Marmite places- love it or hate it.
I could have filled a book on Cumbria and have left out from here far more than I put in. There are many more Churches could have been mentioned, as well as Walney Island.
Did anyone mention Castlerigg Stone Circle at Keswick, or Threlkeld Mining Museum- 2 other places hidden in plain sight?
Rick could easily do a full MNT just on the unknown parts of Cumbria and a whole season on England as a whole!! But that wouldn't be fair to the rest of Europe!
Huge amounts of new flood defences have been built in Cumbria (Keswick included). It doesn't stop us locals nervously glancing over our shoulders every time there is a monsoon rainfall event. It just has to be hoped that never again will the County suffer such devastation.

When we visited Wales in 2018, we hired a car and rode around the countryside. I highly recommend it! You come across neat little restaurants on the side of the road and interesting small towns.

A friend of my distant cousin offered to guide us around to see ancestral houses that my family members lived in, in the 1800's. It was magical. One was still a working sheep farm.

• Bodiam Castle in SE England. For me it is the most scenic and spectacular castle ruin I've ever been to. It's only about a 20 minute drive from the Battle of Hastings site, but it doesn't get the traffic it deserves.
• Lewes is about a half hour train ride from Gatwick and a perfect overnight stop after a flight to fight off jetlag.
• I think of that post from a week or so ago a out Europe's 2nd cities. Glasgow is my first thought. Edinburgh gets all the love, but Glasgow is well worth a few days.
• Another Scottish gem is Calgary Beach on Mull. While everyone else is heading to Skye or daytripping to the south side of Mull to get to Iona and Staffa, turn right instead of left off the ferry and stay in Tobermory and have a day at the beach in Calgary. Aqua blue water and fine white sand, it belongs in the Caribbean.

I agree about Bodiam Castle. It's my favorite. We've been there twice, once in a light snowstorm – magical. OK, I also love Doune Castle in Scotland :-)

Dorset has some lovely towns which I hope would meet your requirements for off-the-beaten-path. A personal Dorset favourite of mine is Sherborne which has scenic footways to the surrounding towns and villages, a very narrow high street with nice shops and an ornate eighth century abbey.

And if you come to Devon definitely see Dartmoor which is accessible by train. Exmoor is a bit more difficult to reach but equally worth it