EU nationals who wish to apply for a Russian visa in the UK are now required to provide proof of their UK permanent residency ("settled status"). They must apply to the EU Settlement Scheme: https://www.gov.uk/settled-status-eu-citizens-families
Once you get your settled status confirmed, you must access your confirmation online:https://www.gov.uk/view-prove-immigration-status , print it and get it notarised. Only then you can apply for your Russian visa in the UK and travel to Russia.