I don't know if this counts but there was a thread last week about Hever Castle which I believe is not far from London but I'm not sure if it is in your direction.
Loads of mums recommended it and I'm taking mine there over Easter. They apparently have an Easter trail or something which I'm trying to find out more on but can't access their website at the moment. I also asked about good swimming pools for a toddler and was given some advice (another thread but can't remember title) which you might want to try. The advice was in London but not North. Sounded easy to get to on the Tube though. I'm going back to my swimming pool thread next week to find out where it is.