My guess is that they’re balancing cargo capacity with efficiency.

For the repatriation flights they seem to use G-YMMB-F a lot. These are LGW frames and are in a high density configuration, this allows them to take as many passengers as possible. In fact the LGW 777s have the most seats out of all of BA’s twin engined aircraft.

As for the G-YMM* 777s over the G-VII* ones, I’d say it’s down to crew bunks. Two 777 have actually had all the seats removed to allow for extra cargo, so the bunks will be needed. Also, the ones that take cargo on the seats, have the old and tatty BA cabin so I guess it doesn’t matter if there is damage.