I agree that it will, sadly and tragically, take more terror attacks before Europe really wakes up and does what is necessary to save itself. But I’m not sure about Britain being in the clear now.
Brexit was a good start, though of course it will mean nothing if post-Brexit they still accept free movement from the EU in exchange for access to the common market. But even if they do stop mass immigration from the EU, they still allow way too much immigration from third world nations like Pakistan and Nigeria, and that is more of a threat than Polish immigrants.
There is nothing wrong with restricting immigration from outside Europe. There is a world of difference between a white Pole or Italian from an historically Christian country and a brown Muslim Middle Easterner. Not all of the latter are terrorists, of course, but they are not assimilable. Historically, there has been considerable migration within Europe. Benjamin Disraeli was a Sephardic Jew; Eamon de Valera, born in New York, was one-half Spanish; Angela Merkel is partially of Polish ancestry. However, Muslims are not assimilable unless they renounce Islam, a dangerous action anywhere there is a large community of them. Barring further immigration from outside Europe is a first step. Expelling resident aliens is a good followup. Offering Muslims and other nonwhites who are citizens financial incentive to return to their ancestral homelands is a third.
However, Europe will need leaders like Marine Le Pen and Geert Wilders in power to accomplish these goals.