According to schedule updates by Russian airline Aeroflot (SU/AFL) they plan to operate an Airbus A350 between Moscow and London on five of the weekly flights for the Winter 20/21 season.
The filings show that the A350-900XWB will operate 5 of the 14 weekly flights between Moscow Sheremetyevo (SVO/UUEE) and London Heathrow (LHR/EGLL) from the 25th October.
According to RoutesOnline the the A350 will operate the earlier of the daily flights on weekdays with the remainder being operated by a mixture of Airbus A321 and Boeing 777 aircraft.