Retired..
As others have said it's not a handling/aircraft limitations issue
Just to add to all the previous comments "LHR" have demanded flow rate restrictions which are becoming more restrictive as the day goes through. Currently (AM) the allowed rate is already down to 38 an hour and by late PM it's down to 30.
It's been a feature of LHR ops for a long time now and whilst as DR UK has said Time Based Separation has helped it's not a complete solution. The BA shorthaul flying programme, being mainly LHR based, takes a major clobbering when these restrictions are put in place.