Is it probable that Russia wanted this and BA got something in return?
Probably not.
BA has got away with operating short haul aircraft on this route in the past but its competitors have upped their game.
It hasn't been confirmed which fleets are operating the flights but in the past what has in part stopped BA putting long haul aircraft on this route are LHR Worldwide crew working practices that don't allow them to do a trip out and back in a day. So I assume that unless BASSA agree to a change in working practices the 747 rotation will be operated by Mixed Fleet.
BA may have also been required to obtain regulatory approval to operate a 747 on this route.