Day Trips and short trips from London suggestions please

I don't know if you chose Moreton in Marsh because of a family connection or something else, but I'd just mention that Moreton in Marsh - to me - is not like a quintessential Cotswold village. It is a market town with a train station and everything a town has.

Smaller places like Chipping Campden or Broadway, both larger villages, tend to attract more folks looking for "the Cotswold experience".

FWIW

We visited Bletchley Park a few years ago and it was fascinating! It’s about an hour by train from London.

• Do you have an interest in castles? I thought Warwick was outstanding.
• Salisbury for it's Cathedral. Personally, I think Salisbury is worth an overnight stay, but it can be done in a day. Same for Canterbury, which is in East Kent, so maybe you can find a wine tour from there.
• Hampton Court Palace and Richmond make a nice day trip.

This is a list of Kent Vineyards.
https://www.kentvineyards.com/a-z-index/
Looking down this list Chartham Vineyard strikes me immediately as being one you could do by train.

But most of them you would need other transport from a station.

Brighton would be a good one by train- a very fast and frequent train service by train. With all due respects to Margate I'm not sure why an overseas visitor would want to visit there.
Another good seaside resort would be Southend- an hour by frequent train. Preferably on C2C from Fenchurch Street (right by Tower Hill, the Tower of London) to Southend Central as that has several stations by the seaside in Southend- including Leigh on Sea with it's old harbour and great shellfish, Chalkwell and Westcliff on Sea for the beaches. And the longest pier in the world which has it's own little train if you don't feel like the walk. A traditional seaside resort for London. The Greater Anglia service to Southend Victoria is fine but Victoria is at the top end of town, Southend is beach and pier, no real indoor attractions unlike Brighton. A traditional Londoner's seaside resort. Also well known for it's pie and mash shops!

By Cheltham presumably you mean Cheltenham Spa.

I haven’t been yet but I’ve always wanted to visit Whitsable ( way before the TV series Whitsable Pearl existed).

Give consideration to Eastbourne as well.

Cotswolds best seen with a car. I stayed in an AirBnB ( no longer available) in Winchcombe one December. Liked the light show and tour of Sudley Castle. I drove to Winchcombe from Oxford.

https://www.timeout.com/london/things-to-do/chocolate-box-villages-within-reach-of-london

Thank you very much!

Arundel Castle also makes for an interesting trip and has lovely gardens. 10 minute walk ( or an even shorter taxi ride) from the train station. It has the advantage that it is off the radar of most American tourists but is a good day trip.
https://www.arundelcastle.org/

The other possibility is to go to Portsmouth to the Historic Shipyard there if you have any interest in Navy ships or history.

Really, there are unlimited day trip possibilities. It is really all about what interests you.

How about Rochester - lots to do and see there...

With regard to Cheltenham, in my opinion there is little of interest in the town itself and nothing that can’t be found in other English cities. I found nearby Tewkesbury more interesting. I was in Cheltenham a couple times on business trips and saw everything I wanted to see in a couple hours. Bath makes for a nice day trip. I liked Cirencester the little time I spent there, but am not sure there’s enough to fill a day.

I would combine Cotswolds with Bath and Salisbury. Cardiff is easily reachable from there too. Plan this as one week and have one week in London.

Bath takes almost a full day to explore.
Same with Salisbury.
We spent two days touring the Cotswolds from top to bottom
Oxford and Cambridge are good day trips from London.
also, Canterbury and Windsor Castle.

Wales is too much for a day trip

Arundel also works as a day trip from Brighton, as does Lewes.

I'm not a fan of paying London lodging rates to take a lot of day trips to cheaper destinations, especially when considerable travel time is involved. My preference is to spend 3 or 4 nights out of London in an area with multiple points of interest. There are good one-day bus tours to the Cotswolds from both Bath (Mad Max) and Moreton-in-Marsh (GoCotswolds), which is easy by train from Oxford. Bath works as a base for Bristol (about 15 minutes by train) as well. Unfortunately, lodging is fairly expensive in Bath and Oxford, so decamping to one of those towns from London may not do a lot for the lodging budget.

Whitstable
https://maps.app.goo.gl/yQ296zpxgAJDVLdL7

Mentioned above already. Sept should be timely for oyster.

Other coastal/ near coast towns:
Chichester, Lewes, Bexhill, St Leonard

Southern train day save rover offers good value in covering a couple of towns.