|Some of the detonations|
For ease of calculation, I will be limiting this to Topol/SS-25 ICBMs at 800kt each, going off 1000 metres above their targets - this does more damage, but produces less fallout. This will just cover the initial attack - not fallout, which is very much weather dependent.
The targets are:
Aldermaston (nuclear bomb construction)
RAF Boulmer (Air Defence control)
RAF Brize Norton
Faslane (Naval base)
RAF Fylingdales (Early warning radar)
RAF Lakenheath (USAF)
London (10 Downing Street to be precise)
RAF Menwith Hill (US/UK intelligence and communications facility)
Plymouth (Naval base)
Porton Down (Chemical Weapons)
Portsmouth (Naval base)
That is 15 ICBMs... Unfortunately, the system keeps resetting itself, but I end up with well over a million dead, mostly those in London, Plymouth and Portsmouth, but other areas will produce thousands of dead. I did not cover army bases, but it is entirely possible that the Russians could eliminate the UK militarily with 30 or so ICBMs.
However, the UK would still have a missile boat out in the North Atlantic with 8 Trident missiles and those could do a lot of damage in return... not to mention all the US boats.