Artistic License – Military: While it's probably not wise to debate militaria with a retired general who led troops at Arnhem, common wisdom was that any nuclear strike (such as the attack on Birmingham and the retaliatory strike on Minsk) would result in an overwhelming Alpha Strike response intent on completely crushing the aggressor rather than tit-for-tat strikes, and that it would be seen as suicidal and insane for either the US or USSR to target only one city in a nuclear strike by the late '70s-'80s. In Real Life, that was the key fact that caused Colonel Stanislav Petrov, the duty officer at the Oko nuclear early-warning system, to not report that five inbound warheads appeared to be inbound to Moscow in 1983. Knowing that the United States was not stupid enough to launch such a paltry first strike given the USSR's doctrine of overwhelming response, he deduced an error in the system. He was right.
Moral Event Horizon and Shoot the Dog: The Soviets, getting thrown back in West Germany, decide to try and scare NATO to the negotiating table by nuking Birmingham, close enough to a summit in London for the attack to be seen from there, stating that further strikes will follow unless they agree to talks. The NATO leaders decide the only way they're going to avoid a complete collapse of public morale is to nuke Minsk back.
The Soviets stop peopleleavingMinsk. The British stop rescuers from entering the centre of the city- it can be inferred, though, that since there's unlikely to be any more survivors there, there was no point in rescue workers risking their lives when others needed help.