the inconvenience caused by consolidating flights should be trivial in comparison
Sorry, but you clearly have no idea how the airline industry operates, in particular the competitive advantage that results from providing frequency (one of the reasons, in fact, that Little Red has failed to compete effectively with BA on UK domestic routes).
Airlines could be given a few years to acquire 77Ws/748s/380s for the role
If airlines were compelled only to use wide-body aircraft at LHR the runway capacity problem would disappear overnight, because many of the routes flown by the narrow-body types that account for two-thirds of all Heathrow movements would no longer be viable.