Wales in nice if you like the country side. If you are a cycling fan, you could cycle in Brecon Beacons.
London is good if you like clubs, pubs and theatres and museums/art galleries.
Any time of year is OK.
Centre parks is good for action holidays - they have horse riding, golf, water rides, archery etc. There are three Center Parks in UK. I've been to the one in Norfolk. Went in March.
Southend on sea (Essex) is interesting. The world's longest pier is here, there is an amusement park and Kursaal with bowling, restaurants etc. Brighton is also good to visit in summer, for its pier, and amusement arcade.
There are various castles to see in UK like Kent castle, Blenheim palace, Windsor castle.
Hope this helps!!