From 4 September 2021 the Russian authorities no longer require travellers arriving from the UK to observe a 14 day quarantine period.
The Russian government requires all arriving passengers to obtain and present a negative COVID-19 test certificate dated less than 72 hours before travel. Airlines may require you to show this on check-in and some airlines require the certificate to be no more than 72 hours before arrival in your destination (please check with your airline). You should not use the NHS testing service to get a test in order to facilitate your travel to another country. You should arrange to take a private test.
https://www.gov.uk/foreign-travel-advice/russia/entry-requirements