As I note elsewhere in this thread (!) while Threads might have been pessimistic about long term impact (e.g. nuclear winter) it is actually fairly optimistic about the scale of a Soviet nuclear attack on the UK.
You're certainly right, that at one stage the plans the Soviet Union had for the UK resembled what the UK did to Heligoland.
In the film, if the West had initiated the exchange with a surprise counter-force first strike, they could have potentially disabled a significant proportion of the Soviet Union's strike ability. If this were the case, one of the morals of the film would have been that even if the majority of enemy warheads are destroyed, they'll still have enough to destroy you.
Another scenario is that the Soviet Union went first with a limited strike, and the West retaliated destroying any further offensive capability.
There are also scenarios where counter measures worked, devices were a lot less reliable than expected and so on.
Thankfully none of this was ever played out, but there is a whole spectrum between nothing and everything (and the politics - MAD, et al) to go with it.
From the chapter titled "The Axe Theory" (http://militera.lib.ru/research/suvorov12/05.html):
[A]s I studied American theories of war, I came to an appalling realisation. It became clear to me that a modern American cowboy who is working up to a decisive fight will always expect to begin by spitting at and insulting his opponent and to continue by throwing whisky in his face and chucking custard pies at him before resorting to more serious weapons. He expects to hurl chairs and bottles at his enemy and to try to stick a fork or a tableknife into his behind and then to fight with his fists and only after all this to fight it out with his gun.
This is a very dangerous philosophy. You are going to end up by using pistols. Why not start with them? Why should the bandit you are fighting wait for you to remember your gun? He may shoot you before you do, just as you are going to slap his face. By using his most deadly weapon at the beginning of the fight, your enemy saves his strength. Why should he waste it throwing chairs at you? Moreover, this will enable him to save his own despicable life. After all, he does not know, either, when you, the noble hero, will decide to use your gun. Why should he wait for this moment? You might make a sudden decision to shoot him immediately after throwing custard pies at him, without waiting for the exchange of chairs. Of course he won't wait for you when it comes to staying alive. He will shoot first. At the very start of the fight.