Museums have lots of kid activities during summer. Check out their website.Battersea Park has lots of fun facilities during summer
Golfing in the UK are very cheap compared to HK. In London, there are mainly suitated in zone 4 or further. Some does not require membership.
If your boys like planes, the RAF museum is worth visiting.
Legoland in Windsor is not far away from London. My boys loved it at that age, you need two days there to enjoy all the fun.