They do sub 777s quite regularly however.
BA never sub routes on an ad hoc basis, its simply too complex to arrange 2 additional crews in BAH to operate the shuttle and return sectors. They did change in Dec/Jan from 777 to 747, but its been a 777 for the entire summer season. It is reverting to 747 in Oct for the whole winter schedule not in Dec (which covered the last few NNCW 747s being in Cardiff). Next summer is probably not fully planned as yet so expect some changes.
DXB in summer has 3 x 777 flights and in winter 2 x 777 and 1 x 747.
DXB in summer only has 2 x 777 and 1 x 767 1 x 777 and 1 x 747 in winter.