Okay, I'll bite. 1 vs 1 - Britain would stand a much better chance than any other country in Europe. They certainly couldn't invade us. Why? Technological advantage and naval power.
Forget nukes for a moment, because in truth they could annihilate any nation with their stockpile. Britain would launch back regardless, but in that scenario it's NATO vs Russia, not UK vs Russia.
The Royal Navy is objectively and comfortably the most powerful and well equipped navy on the continent. Russia's maritime hardware is largely decrepit, outdated and often in poor state of repair. Their aircraft carrier literally requires a tugboat escort at all times, their submarines are in many cases technological dinosaurs and inferior to ours by every meaningful metric and comparison - and their naval air power is certainly no match for that provided by the RN's new carrier class, backed up by the RAF at shorter range.
This would make a land invasion of the UK very, very difficult. They'd need a huge armada to send tanks or soldiers in any meaningful numbers, and that many targets would constantly be harried and sunk by relentless British air and sea bombardment, their losses at sea would be so catastrophic that it wouldn't be worth even attempting - even from multiple directions, the combined heat they'd face from the Royal Navy, the Royal Air Force and the Fleet Air Arm would make it a non starter. Whatever air carrier power they had would be hopelessly outnumbered and destroyed in early air to air exchanges. They may attempt long-range air patrols from land to provide cover for this 'mega flotilla' but the sheer distance between Russian airbases and the English coast would make this almost impossible in any effective sense, even if all of their fighters were crammed into Kaliningrad, they'd still have to make 2,000 mile round trips 24/7 to provide constant air support at sea, whilst coming up against technologically superior aircraft and better trained pilots.
No, Russia could not 'crush' the UK in any conventional sense. Again, a nuclear war is different - then again, Britain at least has a nuclear deterrent. Regardless, a nuclear exchange with the Russia = NATO vs Russia, so we're not the only ones being crushed.
Most historians agree that even if Germany had not been defeated at the Battle of Britain, Hitler would never have been able to mount an invasion of Britain for the exact same reason; naval superiority - which remains to this day, Britain's strongest military asset.