Nor do you say if you have kids...

I've found it worthwhile gettig an English Heritage annual pass, which gives free access to many historic sites within easy range (nearest being Eltham Palace). Then we got a Historic Royal Palaces pass, which gives access to the Tower of London, Hampton Court, and Kensington Palace, among others. We will probably get National Trust membership next.

If used wisely, these passes can save lots of money over a year (though make sure you do use them, or it's a false economy) and give you lots of good ideas for inexpensive days out.