If you are remaining airside and not entering Russia, and have proof of onward travel to show the gate agent, I don’t see why they shouldn’t board you. Don’t need a visa if you don’t plan on entering the country.
Edit: I suppose you and BA could end up in trouble if they had to divert to another Moscow airport for weather – you would have to enter Russia then. This causes issues sometimes when aircraft divert mid-flight back from Asia and land somewhere like Kazakhstan – a 777 full of passengers is suddenly stuck in Almaty or something and the airline can’t do anything for them in terms of accommodation because they can’t leave the building.
This has come up before, hence my post - a previous FT member was denied boarding in LHR on a BA b2b because it is not ‘in transit’ - they have become strict on this point so it is not even worth trying, if you want to try a b2b with BA you will need a valid Russian visa.
(In transit has become like the Chinese version, point A-B-C, not point A-B-A)
Pilot37