That raises an interesting point that was not covered in the plot of Threads - the strategy behind the nuclear exchange and how it played out globally.

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.