Any ideas for a location for a self catering holiday with a 2.2 year old and 6 mth old in Feb/March?

[MummyToOneForNow]

Ideally within about 3hrs drive of London with indoor things to do with dd if weather is bad... Dh will have holiday to use up before the end of March and we want to go in term time while we can but don't know which areas might be good for things to do. We have been to the Cotswolds but most child-friendly activities seem to be outside.

Dd seems to have phobia of swimming pools and dh not that keen on the idea of centerparcs so that option is out...