It remains the case that, whilst effective but not unduly overbearing policing and secutiry measures wuld have helped and will still do so, the fact that people based in the West - and not just first generation immigrants thereto - operate largely undercover from within Western countries as part of terrorist networks that are headquartered elsewhere; internet, email and other technology enables this in ways that would simply not have been possible just a couple of decades ago.
Anywhere might therefore be or become susceptible to the activities of terrorists; as to the areas of UK arguably most vulnerable to potential terrorist atrocity besides London, Edinburgh, Cardiff and Belfast (the seats of government) and major cities such as Birmingham, Manchester, Liverpool, and Glasgow because of their size and population density, perhaps the most obvious targes might be Cheltenham (home to GCHQ) and just up the road from me in Hereford (home to SAS). Glasgow, incidentally, has just become the venue for the fatal stabbing of a Muslim by another Muslim in punishment for having published Easter greetings to his Christian customers on Twitter; one might recall the old cliché once uttered by those who dislike the Glasgow accent that when a Glaswegian greets you with "good morning" it sounds as though you're being accused of something, but taken to disastrously criminal ends, in the sense that one had better be very careful what one says to whom or publishes for the benefit of whom in Glasgow...
Of course population movement has its part to play in enabling terrorism to thrive, but it's by no means the only consideration. By the way, someone recently joked on a BBC Radio 4 programme that sends up the previous week's news that it seemd daft for ISIS to spend so much money on surface-to-air missiles when they could buy up job lots of taser guns at the local supermarket for a few pounds apiece; I'm rather surprised that he was allowed to get away with that, even as a joke (and I've never seen such weapons on sale at my nearest supermarket)...
Best,
Alistair