Other than the Irish, who are a sort of exception to the rule, the last group to be genuinely persecuted by an oppressive regime in the UK were the Jacobites in the 18th Century. Albeit, right until the early 19th century, Roman Catholics and Jews were effectively second-class citizens, but the serious, official persecution of these groups was over and had been for some time.
What's more, even for these out groups, the oppressors have been our own people, not foreigners.
In living memory, and indeed in the furthest memory of our great-great-great grandparents we have not been occupied by a foreign power. This is almost unique in Europe, and most of the exceptions (e.g. Spain, Portugal) have had deeply oppressive internal regimes easily within living memory.
I think it is a factor - though not the only one - in the rather special way we look at ourselves. Indeed, nostalgia for our 'finest hour' when we (aided by various foreign allies) most recently kept the wolf from the door remains very strong, almost totemic, even though only a small and diminishing minority were about in those days.