Camper rentals in England

18 hotels in 21 days does sound exhausting. To encourage an average stay of 2 nights, one tactic may be to arrive at your overnight destination rather late, so you are focused on exploring that town all the next day, rather than feeling that you already saw it the previous afternoon. Also include a few larger cities or home bases where you can find a few days worth of sightseeing.

I've never tried the camper route, but certainly would like to. See tips at https://www.ricksteves.com/travel-tips/sleeping-eating/camping.

Also, without knowing your particulars, I'd say that a retired couple usually has plenty of time, so stretch the trip as long as you possibly can.

Pros

Many British camping grounds are stunningly beautiful.
Many of them are also very well-equipped with stores, fancy bathrooms, restaurants, and even swimming pools.
It's a great way to get to know people as many Brits love to camp during the summer months. We have found people in campgrounds there to be very friendly!
Cons
Gas in Britain is quite expensive. That, combined with the cost of the campervan rental and the campsite, can be more expensive than staying in hotels.
Many roads in Britain are quite narrow and can be challenging to drive, especially when driving on the opposite side of the vehicle AND the road. Totally depends on where you plan to go though.
Overall I would say that using campgrounds in the UK is a great way to travel! It will give you a very different experience than what you would have in hotels. If you are interested you should give it a try, maybe even if it's just for part of the trip?

We've been to Wales several times and I can't imagine driving those narrow roads in a campervan. We were so glad we took Rick's advice to rent the smallest car we could fit in (and bring minimal luggage, i.e., one carry-on wheeled suitcase and backpack per person). My advice would be to rent a car and stay in fewer places. Spend at least two nights in each location. Then you will not be so tired.

we spent 3 weeks or so last year driving around parts of England/Wales. I thought the roads in Italy were narrow !!! Wow.
While I have driven and used campervans I could not imagine enjoying a campervan driving holiday in the UK. It was bad enough worrying about scratching the sides of the smallish hire car on the narrow roads. and often having to back up to let people pass.
The numerous camping grounds that we saw were not much more than a paddock in someones farm ( though I am sure there are plenty of great spots) we were just amazed at where caravans/campervans were 'holidaying'.

Try not to bite off too much as its surprising how long it takes to drive around. and perhaps use some towns as a base,so you are not moving accom so often. with only 3 weeks to do all you mention you will just see plenty of roads and not much else.

We very much enjoyed staying in B&B's/guesthouses etc. you get to talk to locals and other travelers and the car hire was not too expensive.

hope this helps.

Is there some confusion in your terminology? A campervan in the UK, such as the old VW model doesn't have any bathroom facilities and is very compact e.g. the seat becomes the bed at night. A motorhome is larger and does have a bathroom. I presume you want to hire a motorhome? Three weeks isn't anywhere near long enough to cover 4 countries, so I suggest that to start with, you drop Ireland is this is the trickiest to get to! I had 28 nights towing a caravan in Scotland last year and we only just scratched the surface here, as we didn't want to be driving miles each day. I recommend combining England with either Wales or Scotland, whether you are motorhoming or driving to a hotel/cottage/B&B.

It is illegal to wild camp in England and Wales (not in Scotland, although the Scots are trying to change this), so you need to be on a campsite each night. When are you travelling? Sites in popular places, such as Skye, get booked up months in advance for travel
between early June and the end of September, so you are unlikely to be able to just turn up and camp. The costs usually work out slightly cheaper staying in cottages/hotels/B&B vs hiring a motorhome, but it depends on when you travel and how up market a place you need. Petrol and diesel in rural parts can be expensive here - it's currently c £5 a gallon in major cities for fuel and add say 10% for rural places. Anticipate on getting c 30 mpg in a motorhome, less if you are in hilly parts. Compact cars here can average 65 mpg + on longer drives.

Are you comfortable driving a manual transmission? Motorhomes are not automatic. Some of the roads in rural Scotland and Wales in particular are very narrow, many with passing places and you need to be OK with manoeuvring a large vehicle in tight spots.

Whether you are in a motorhome or going down the hotel/B&B route, it isn't a good holiday anywhere to move to 18 places in 21 days - no wonder you were exhausted! In many parts of the UK, there is sufficient to keep you occupied for at least 4 days, if not a week or more in each location, so a rushed pace would be moving on every 3 nights. Ideally move every 4 nights, then you get to appreciate something other than looking at a tarmac road. Although places don't look far on a map, if you are travelling on motorways, there are often crashes that delay traffic or the sheer volume of traffic means that you move slowly. This applies whether in a car or motorhome.

If you are covering two very distinct geographical areas such as London and the south coast of England and the Scottish Highlands, consider taking the train between the two rather than driving.

It's difficult to advise further without knowing more about what you are interested in and where you want to go.

I want to thank you for the responses. We're still deciding the best way to proceed but now I at least have more information to consider.
rsa-rla