"At one point in the film, Helen Mirren boards a red RM to go and visit her daughter in a mental hospital. By the time she arrives at her destination, the bus has miraculously transformed itself into a red MCW Metrobus!!! " noted by Steve Phillips, but a review of the film shows the director intended to show two bus trips.
Firstly Mirren boards RM1280 (280CLT) - when filmed this bus was in preservation:
She's seen sitting aboard the RM, and then another bus.....
....which turns out to be MCW Metrobus M176 in London General livery, although it appears to have been sold by London General before filming: