Day trips

Did you check this website: https://www.nationalrail.co.uk/live-trains/?

stations close after the last passenger train has departed or the last arrival has arrived.

If you want to know about a particular tube station the TfL planner app and website have detailed info on stations including first and last train times. Figure about midnight or a little earlier at most except for when the Night Tube is operating.

National Rail Enquiries, where you plan you train travel, has information about every station on the network including both opening hours and staffed hours (if any - some are unstaffed).

Well for instance Brighton has frequent trains until very late, with the last being at 0002 (2 minutes past midnight)- a Thameslink- so central London stations at Blackfriars (arriving 0136) and St Pancras.
The last to Victoria is at 2239, arrive 2348 which should be in time for the last tubes.
From Whitstable last train is at 2228, back into Victoria at 0034.
There are always taxis back to your hotel if after the last tube.
And a very good network of all night buses. Nowhere a tourist would be staying will be very far from a night bus stop if you have missed the last tube.
For instance the N15 runs all night from Blackfriars to Aldgate (and the N11 from Victoria with a long shared section to change onto the N15) and the N205 from St Pancras.

I just did a day trip to Whitstable last month! The train from Victoria Station worked well. I guess I could have spent more time there, but I think making it just a day trip with a reasonable departure in the AM and return late afternoon/early evening will do you just fine without trying to stretch the day too long.

Thank you all for your responses. Great info and I will definitely need to get some trips booked! Thank you so much.