Elmo,
Any war in the Central Region was expected to go nuclear with tactical weapons. The sheer number from mines, artillery, Long Johns up to SS-20s made it almost inevitable. The normal exercise/TACEVAL last 3 days with nuclear exchanges on the 3rd day as Red forces approached the channel.
The British aim, unsurprisingly, would have been to limit use to the european mainland, with no use against the UK itself.
Presuming the aim of the USA and USSR would be to limit the war to Europe, including the UK, requirement was therefore to couple the use of any such weapon against the UK to the initiation of a general exchange including the USA and USSR.
The USA might not be willing to launch against the USSR just because London took an SS-20. But if the USSR hit London, the UK had the ability to hit Moscow (the purpose of the
Chevaline upgrade to maintain the Moscow Option). And a Polaris launch against Moscow from mid-Atlantic would inevitably force the USSR to launch against the USA and the inevitable retaliation.
Both the USSR and USA therefore had a sizeable incentive to ensure no nuclear weapons were ever launched against the UK, and that drove up the risk of starting a war in the first place.
Flexible Response might have replaced Tripwire as NATO policy - but the UK still had it's own wire in place*.
*
As described, the strategy of flexible response did "not specify the precise nature of NATO's reaction to a particular attack. It [had] been argued that the ambiguity enhance[d] deterrence by complicating Warsaw Pact planning."4 Ambiguity was also needed in strategy so that the US and Europeans could interpret the strategy to suit their own views. This difference of views within NATO was concentrated primarily on the employment of tactical nuclear weapons. In general, US strategists saw a deliberate and prolonged conventional defense in Europe. Further, they saw the use of tactical nuclear weapons as a way to keep a conflict from becoming a strategic nuclear exchange between the US and the Soviet Union. On the other hand, European strategists desired to see a brief conventional defense phase and viewed the use of tactical "nukes" as a means of coupling US strategic nuclear weapons to the defense of Europe. This coupling would preclude only Europe from becoming a nuclear battlefield.