Olmy, you're referring to the North stands which serve Gates 57 etc (UK Domestic).
You're right, more than one aircraft (if both are small) can be parked on one gate - that's because they are designed for large aircraft and have been adapted, rather than the reverse.
There would be some areas in LHR (T2 cul-de-sac, T1 Domestic) where even parking (never mind connecting to facilities) anything larger than an A320 would cause serious disruption.
Longhaul 777/747 aircraft are confined to T4, T3 and certain T1 stands only - and they're all full.