A few things work in our favour here, notwithstanding the fact that, as you say, the terrorists only need to be lucky once.
(1) It's a lot harder to get hold of firearms in the UK, specifically the automatic sort favoured by the Paris attackers, due to (a) our very tight gun legislation and (b)...
I won't even bother saying "let's avoid speculation" as inevitably, we're already some way down that road.
But casual/ outright racism, xenophobia or unpleasant religious bigotry won't be tolerated.