The argument of using B757/763s on the shuttles 10 years ago, vs A320s nowadays, a lot of that has to come down to the improvements made in the railways, more and more people are choosing to take the train down to London from Manchester than fly, hence the smaller a/c ?
Of course, people are still using LHR as a transit point hence why I believe BA are adding an extra LHR rotation from MAN in place of a LGW flight from the winter schedules as well