Day trip from London

What time of year? Oxford and the Cotswolds are lovely in the spring. Easy to include Blenheim, too. You can do that by bus and train without a tour.

Thanks for the suggestions! I am going in a couple weeks - so mid-March. Probably not optimal weather but hopefully fewer tourists, all things considered

I would do Bath over Windsor. Loved the Roman Baths, take a walking tour, Museum of Costume, Bath Abbey.

Pre-pandemic, I typically traveled to London mid-March. It can be sunny & rainy; it’s England. You have all sorts of choices depending on your interests — castles/palaces, colleges/universities, cathedrals/churches, museums, villages, cities, etc.

That tour does sound rushed and very long - all at the same time. I’m a Jane Austen fan, so I’d pick just Bath. It’s very doable on your own by train from Paddington Station. The Roman baths and the fashion museum are walkable from the Bath station. Another less ambitious, close-in trip (that I would consider more like a 3/4 day) is Hampton Court Palace.

Windsor or Hampton Court Palace or Ham House. Research all 3 and see if any generate interest.

Trying to see Stonehenge, Bath and Windsor in one day, would be insane.

I agree with Claudia and others, that 12-hour tour tries to do too much. I suggest a day trip to one destination, and there are many choices. Bath, Salisbury/Stonehenge, Windsor, Hampton Court, Bletchley Park, Oxford, Cambridge, Canterbury, Winchester, and Brighton come to mind. Are you interested in castles, cathedrals, colleges, prehistoric sites, Roman sites, seaside scenery, something else?

Bath is about 1.5 hours from Paddington each way and offers a Roman bath, a late Gothic abbey, Georgian architecture, and the Jane Austen Center. All that would be a busy day but you could prioritize. Salisbury and Stonehenge offer a Gothic cathedral and nearby prehistoric site. And so forth. Here are our host's descriptions of places he recommends to visit in England, some of which could be day trips from London. His choices are good but not exclusive -- there are others. Look at these and a guidebook or two, consider your interests and energy levels, and come back if you want with more specific questions.

That tour sounds exhausting and would not give any of the places enough time.
You do not need a tour for Windsor Castle. Take the train almost the whole way there and then transfer to a shuttle train. Very easy. We bought tickets ahead online thus avoided what was a very long line to get in.

I prefer my day trips to be spent seeing places rather than spending hours in transit. Covering 3 places in one day means barely enough time to get off the coach. Whilst the train to Bath can take just under 1.5 hours, depending on where you are staying, you may need to leave another hour plus to get to Paddington station and to find your platform. I think Bath is better with an overnight stay. Trains are usually cheaper with travel after 9.30 and if booked 10 weeks in advance, which isn’t an option for you.

I would therefore opt for Windsor of your two options. I would also consider Oxford, Cambridge or Brighton, depending on your interests. Hampton Court is another possibility.

I suppose my first question is - are you looking for any day trip, or are you restricted to day trips that involve those destinations?
There are a few others I can think of -

1) Ely & Cambridge. Take the train from Kings Cross to Ely in the morning. Walk up the hill and see the cathedral and Oliver Cromwell's house. Have lunch at the Cutter down by the river and take the train back 15 minutes to Cambridge. Once there, walk into town (or local bus if you prefer, its right at a mile) and see the architecture and museums of Cambridge. Stop by the Eagle and see the RAF bar, have supper, and take a liesurely train back to London

2) Canterbury & Dover. Take the train to Canterbury. Walk to the cathedral and castle. Have lunch at Pinocchios. Back to the train and head to Dover. Walk to the castle. See the castle, and make time for one of the last entrances to the Secret Wartime Tunnels. Grab a quick bite and train to London.

3) Hever Castle & Gardens (Anne Boleyn's childhood home). This one is great if you like walking, as its roughly a one mile rural walk from the station to the castle. The website even has a quick video on how to do it.
https://www.hevercastle.co.uk/visit/opening-times-directions/

a 12 hour in March will partly be in the dark. Sunrise around 6:30 and sunset around 6. You'll either start before dawn and/or finish after dark.

and you will be dead tired and probably not have seen all you hoped to.

We did Stonehenge as a day trip from London, and it felt quite rushed. Next time, I would rather do that as part of a driving tour of the West Country.

Bath is even farther.

Windsor is an excellent day trip. You can get there by train in about an hour from either Paddington or Waterloo. Once there, go to Windsor Castle first. Its really nice, especially the Chapel, with the banners of the Knights of the Garter. Then walk across the river and take a tour of Eton College. Its a beautiful place, and so many famous people went to school there. We saw a desk that Percy Bysshe Shelley had carved his name into!

Other ones that I've never done, but I know Londoners do, are daytrips to the seaside, at Brighton or Southend, both of which have direct trains (from Victoria and Liverpool Street respectively).

“That tour does sound rushed and very long - all at the same time. I’m a Jane Austen fan, so I’d pick just Bath. It’s very doable on your own by train from Paddington Station. The Roman baths and the fashion museum are walkable from the Bath station. Another less ambitious, close-in trip (that I would consider more like a 3/4 day) is Hampton Court Palace.”

Yes, exactly what I was thinking - too long but also rushed. Thank you for the suggestions!

Doing Stonehenge, Bath and Windsor Caste in one day is too much.

When we visited Windsor Castle, we spent about three hours there, part of that time was waiting to enter, then go through the lines, etc. You could do Stonehenge and Windsor in one day, but Bath needs a full day, perhaps a bit more.

Day trips to Oxford, Cambridge, Canterbury and Salisbury or Winchester are good.