First of all, you'll have a lot of time. Proxy attack against HMNB Clyde, then at least an hour when the last Vanguard launch his missiles and they hit (or not hit) St. Petersburg, and then few hours until the Russian limited counter value strike. So, if you have a car - you can try find more or less safe place (if the traffic will allow evacuation).Lets hope it doesn't go that far, Manchester is only 20 miles from my Home, so if it ever came to that i wouldn't even have time to make a cup of tea before the blinding light of a nuclear strike then the blast wave not long after hit that would wipe my area off the map.
Second - if they use 100 kt warheads over Manchester - at 20 miles it hardly broke glasses in windows (especially if you have modern windows). You'd better have some water, food, tape to seal the door and windows, some respirators, dosimeters and analog radio (which support frequency 149.200, to call Volga if necessary).
I also hope that Starmer will be reasonable and will accept the defeat right after "Irish/Houthi/Iran/whoever else" attack against HMNB Clyde, but, it seems that he is not really different from Zelenskiy, and will sacrifice millions of British people without any practical result. You know, the members of Cocaine Coalition have really specific mindset.