Originally Posted by
highpeaklad
Legoland is in Windsor I believe
Yes it is, you actually get a view of Legoland from the right hand side of the plane when 09L is used for arrivals.
To the OP if you arrive into Heathrow without any sort of a delay then Windsor Castle or The Science / Natural History Museum in London are all do able.
With travel time and actual time spent at one of those places the 6hr layover time will quickly be eaten in too.