Pre-emptive cringe essentially means backing off without even initially putting up a fight (i.e., you're cringing away from a fight pre-emptively). I know it was coined by Kelly specifically to describe the British Foreign Office, but I can see that it's managed to leak into other contexts as well (for example, here's an [article] (https://www.jstor.org/stable/1501459 ) from an education journal that uses it to say educators don't stand up for themselves enough, and here's a National Review [article] (https://www.nationalreview.com/david-calling/preemptive-cringe-david-pryce-jones/) accusing the Obama administration of doing this with regard to Afghanistan and Israel). In its original context, though, John Kelly was criticizing the Foreign Office for having (as he saw it) a policy of appeasement before demands had even been made. As it's described in the National Review article I mentioned, it was characterized by:

[Foreign Office] officials whose careers had been devoted to internalizing all the bad things ambitious foreigners charged them with, and consequently devising how best to haul down the flag. They were so long accustomed to appease and surrender to strength and violence that they couldn’t imagine anything else.

The book is arguing that this had been the default policy since the Suez Crisis, a major humiliation for Britain, which had been an extremely powerful power for the century before the war. The fact that it (and France) had essentially been strong-armed by the US into retreating from the Suez without succeeding at their objective of reasserting control over the canal was hugely embarrassing for both powers but especially Britain. The whole incident made Britain look like a power in decline that now had to take marching orders from the Americans. The author of the book is arguing that this incident led to a "policy of pre-emptive cringe" because it created an attitude of fear of further humiliation. Since you can't be humiliated by losing a fight if you avoid the fight altogether, the Foreign Office simply refused (in this view) to defend its interests. Before Thatcher's election, Britain refused to join the US and other allies in invading Vietnam (though many would argue that this was, in hindsight, not pre-emptive cringe but just a smart thing to do) and stopped putting up a fight to parts of its empire that wanted independence (Sierra Leone, Jamaica, Uganda, Malta, Kenya, Zambia, Guyana, Botswana, Barbados, and a whole slew of other countries all became independent during this time). Compared to the US, or even France under de Gaulle, which was very active in Africa and got into such a massive disagreement with the US it actually pulled out of the military parts of NATO, Britain's foreign policy could be considered fairly passive. Though, as a counterpoint, I'll say that a lot of people consider European colonialism to be an inherently exploitative and terrible system that Britain was right to relinquish at this point, especially considering that the European countries who attempted to hang onto colonies eventually ended up having to grant independence anyway and sunk a lot of resources into wars that they ultimately lost (to say nothing of the human cost).

The book is comparing this attitude to Thatcher's decision to go to war for the Falkland Islands. Rather than cringing away from the fight before it even began, as Kelly believed previous governments had done, Thatcher defended the Falkland Islands with force and ultimately won the war.