Compare it with another popular leisure activity in this country - supporting a premiership football club:
Most spotters are thrifty at best and don't really want to spend [much]. Airports are businesses and shareholders and owners need to see a return on capital invested.
The reason why there are huge stands at football grounds like Old Trafford is because football fans will shell out hundreds of pounds a year, or well over £20 per game, for a couple of hours of entertainment each week. LHR is in the "premiership" of airports, but even if the facilities were built how many spotters would shell out hundreds of pounds for the ability to sit and watch airplanes week in week out, how many spotters would pay well upwards of £20 per person for a couple of hours or even a day using good viewing facilities - plenty moan at paying a few quid for parking, or go elsewhere to avoid such charges, and even when they do pay they bring butties rather than use catering facilities which would also generate ROI.
So, I suspect its over as far as official facilities at LHR are concerned - OK, the visitor centre was in the wrong place, but any replacement is really not going to generate the return any business needs to justify investment.
A