a quick look a the OS map shows that to build 2 or 4 new runways immediately west of LHR would mean th demolition of Colnbrook, Poyle and probably Datchett plus the removal of some of the reservoirs
You would effectively be putting it in Slough and Windsor (home of our beloved Queen of course) and moving the noise big time over the Maidenhead, Reading, Bracknel triangle
There are some very very rich people around there who would fight this tooth and nail - it would never happen - all teh terminals exceot 5 would be inteh wrong palce and you'd have to spend a fortune on ne w roads - cheaper to go to Boris island and start with a clean slate