Favorite Small Town near York, Cambridge, Oxford or London

Assuming you don't want to just spend that time in either Oxford or London (which you've only alloted two days to), I'd recommend Durham.

Less than an hour north from York by train, with a beautiful cathedral, a castle (part of Durham University's University College) and beautiful historic streets, lovely river, great countryside walks and a market. And there's also Beamish, a huge outdoor museum: https://www.beamish.org.uk/. You could take the train straight back to London (Paddington) from there.

Durham works.

Cathedral is gorgeous. If you’ve watched the excellent Inspector George Gently series it’s where Bacchus was shot.

Lovely river walk.

Durham Market Hall.

Cup of coffee, pot of tea, or a meal at River View Kitchen.

Lastly, the Ye Olde Elm Tree pub is a pleasant respite for a pint.

I took the 3 hour train ride from London’s Kings Cross Station to Durham. No changes.,

Durham would be good , I'm headed there for the first time this coming Autumn . Another possibility would be Whitby , easily accessible from York

Durham is wonderful but I'd hardley call it a small town...

Assuming you really do want a small town then I would suggest something a lot more rural. Think about places like Helmesley or Pickering from York. Both are on the edge of teh North York Moors and have a ruined castyle. Both have a good range of shops and possibility to get out into some superb countryside . Pickering also has a steam Railway too for a trip to Whitby!

Re- Simon's post above. There is no train direct to London Paddington from Durham. All trains go to Kings Cross.
If you wanted Paddington you would have to catch a cross country train to Reading and change there, or more sensibly KX then tube.

Thanks everyone for your suggestions. I'll check out all the towns mentioned.

Ely near Cambridge. It’s officially a city, with a magnificent cathedral, but has the feel of a small town. Very easy on the train from Cambridge.

If you only have 2 or 3 days at each, I would probably stay at the bigger place.

Potential for York might be somewhere like Boroughbridge or North Allerton. Potential for Cambridge would be Ely. For Oxford maybe Stratford upon Avon

jgdh - Am intigued by you suggestion of Boroughbridge or Northallerton for York ... Perhaps you could elaborate on why these places please?

Also not convinced by Stratford on Avon as an option for Oxford (unless they really want to immerse in the Shakespear experience as there are so many nicer places in the Cotswolds), but do agree with Ely...

Burford, Maddingley, Harrogate, there aren't any near London but St Albans might do.

Or in the order you asked - Harrogate, Maddingley, Burford (or Witney, Oxon for the train), there aren't any near London but St Albans might do.

wasleys

Boroughbridge - standing stones (including one larger than anything at Stonehenge), close to Aldborough with lots of Roman history, lovely riverside, great market square, not too far from Fountains Abbey

North Allerton is well-placed for North York Moors and Yorkshire Dales, good old town with all that entails

Stratford upon Avon also has the advantage of a train station and has a lot more in one place than the couple of Cotswolds train furnished places

As said, I would just stay in the larger places originally listed

For Oxford I might look at staying at somewhere like Abingdon, or maybe Banbury.
There are also several small towns with stations on the rail line between Didcot and Oxford, all on the banks of the Thames, and all very attractive.
I know Abingdon has no rail station but has a very good bus service into Oxford.
By public transit Oxford from Stratford isn't impossible, but is far from easy.