using 5 or 6 A380 on this route would please the environmental lobby no end.
Yes - but nobody else. On short business routes (LHR-AMS or FRA are prime examples here), frequency and convenience trump everything. Do you really think people will want to spend 1 hour boarding for a 45 min flight? Don't think so.
On long haul the situation is different.
PS: quite a few high-density routes ex London will probably become partly obsolete soon anyway, once the new Eurostar trains are introduced (the ones that can travel through to AMS and FRA). Much like London-Paris today. The train is simply quicker.