AFAIK, there are/were 2 airlines from each country on LON-MOW each doing 2 flights/day:
BA LHR-DME
BD LHR-DME
SU LHR-SVO
UN LHR-DME
The 2 BD flights are up for grabs with VS chasing, with VS on the route instead of BD, that still leaves 2 flights/day each on 2 carriers from each country.
The UN flights use LHR slots leased from BD, an agreement which BA inherited. When this agreement ends, does BA have to surrender those slots permanently to UN, just generally, or do they revert to BA?
Is BA's LGW-SVO idea hedging their bets in case VS only takes up 1 of the 2 daily frequencies up for grabs?
If so, think VS may regret taking up just 1 of those frequencies, it should be a high yielding route for them.
IIRC wasn't BA doing more than twice daily to Moscow a few years ago?
BTW, find the proposed return to SVO surprising because BA switched all flights from SVO to DME some 8 (?) years ago, must have been good reasons.