The day the BAA provide decent observation facilities at their aiports will never come. All they aqre interested in is making money. Shops and parking are their priorities, not enthusiasts and casual visitors.
Although I have never been there, the fact that the visitor centre at LHR is South Facing put me off immediately. Taking pictures or "spotting" into the sun (if there is any!) is a non starter.
IF, and it's a big if, they do consider a replacement centre, then Southside somewhere in the vicinity of Myrtle Avenue would probably be better. However, as I have already said, I think the chances are zilch.
skiddy