In this day and age, assuming their arsenal worked as intended - which is debatable, the UK post a Russian nuclear strike would look very different to what most people seem to think, and in some ways more hellish. It isn't going to be a sea of radiation. While some warheads would be used against capital cities/high population centres most will be targetted at military and civilian infrastructure, aside from against bunkers/command centres most would be configured for airburst use which significantly reduces fallout.
Reality would be 100s of thousands dead, no power, no ports, mass civil crisis/disobedience, no supermarkets, people fighting for scraps, etc.