I and my husband will be traveling from Canada to Russia and back through London Heathrow (long layovers both ways). We have Canadian study and work permits and we are Russian citizens. Normally, with a Canadian permit, we would be able to leave the airport without a transit visa and go to London if we have a second leg of the flight within 24 hours. Does this rule still apply during COVID times or was it changed?
TWOV is still applicable to your circumstances. However, if you enter the UK you will need to follow the rules on quarantine. Neither Canada nor Russia are currently on the list of exempt countries. https://www.gov.uk/government/publications/coronavirus-covid-19-how-to-self-isolate-when-you-travel-to-the-uk/coronavirus-covid-19-how-to-self-isolate-when-you-travel-to-the-uk