Bicester

Whose grandmother?

Train from Marylebone station.

https://www.travelaboutbritain.com/oxfordshire/bicester.php Hopefully this link will help you decide.

There is the outlet mall (quite a lot of shops, a few restaurants, lots of tourists, prices not low), a fairly standard town centre, and a fairly large military presence.

Oxford is a very short train or bus ride away.

Much of the old town, perhaps where (her??) grandmother lived is still there.

Be sure you know which of the two stations your train will arrive at.

You will be taking the train?

There is Oxford, what a great town, try attending an Evensong at Christs Cathedral, Bletchley Park is very near.