There are strategic assets all over England. Air bases, power stations and so on. I would think Russia would eliminate targets here as well as the US to negate us as US allies.
The US has assets here in England, too. Squadrons of fighter jets, missile defence systems. It's entirely logical to me. If I were to launch a nuclear attack I'd make sure there could be no retaliation. Or minimal.
Electro magnetic pulse from a thermonuclear explosion would out electrical devices and if several nukes in the Megaton yield were used at different locations then the EMP would overlap. Total 'darkness'.
If they use anything like the 50 M/T Tsar Bomba then there'd be absolutely immense devastation. The Russians do have an ICBM that travels at 8km/second. Yep. Per second. And it carries a 47MT warhead.
Likely they'd use subs to surprise attack us after their ICBM's were in flight to the US. And the subs would carry SRBM'S with a cluster of warheads in each SRBM probably in the low K/T range. Just enough to to obliterate airbases.
Transportation would fail. People would be wandering around with burned out retinas if they were facing a nuke when it went off. 3rd degree burns too.
London would be hit. Canary wharf, houses of parliament and downing Street by SRBM from the subs.
Our infrastructure would completely and utterly fail.
This is why atomic war hasn't happened yet. You're either 100% in or 100% out. And that means taking out allies of your enemy.
If there was to be a nuclear exchange it'd escalate rapidly and result in M.A.D.
If you do survive, the land and waterways would be highly radioactive and assuming you didn't get a big dose of rays in the initial blast.
I'm not sure which missiles would be ground detonation or airburst. Ground detonation would result in much higher levels of contaminated earth and debris.
So with the outright devastation and speed at which Russia could inflict it on us, we'd be dog meat.
Think happy thoughts.