SU don't nightstop in Heathrow. Even the 0830 departure to Moscow is a 0730 arrival from Moscow.
Visa regs are not a specific pick at BA but are a nuisance for everyone. Attention is currently focused on Ukraine who recently discarded their old-style visa requirements. The tedious regulations are not new, nor a throwback to the socialist period, they were like that in Czar days ! I wonder what the other nightstop operators at moscow do.
I reckon if the visa controls went (or became straightforward) BA, Aeroflot, and the rest might double their traffic in a year and the Russian economy benefit from huge tourist income.
The British are currently held in high regard in Russia at the moment as a result of the submarine rescue. Time for BA to have another go while the mood is good.