We have stayed at Wyatts Covert at Denham Green 2 years ago. Although it doesn't fit your central london criteria it is so easy to get to central london from here. We got a family day rover ticket from Denham Green railway station it cost £21 for a family of 4 and it included travel to London and back (about 20mins) and tube and buses for the day- bargain. It may be a bit more now but I can't see it being a load more. Also legoland is a short drive away. The minuses are: there is nothing for kids on site and the metropolitan police helicopter can be stationed at the adjacent aerodrome so it can,if it's there, take off at anytime day or night - made me jump when we were there!!
dave