Given how keen they are to recruit, I can’t imagine the infiltration itself could be very difficult, and when we’re talking about infiltrating UK-based groups the dangers are minimal. Terrorists’ strengths are their ability to strike powerfully from a weak position, but we should remember that they are indeed in a weak position. If there is a hint of danger of discovery an infiltrator could be moved to a safe house with minimal fuss, a terrorist’s strength is their ability to strap a homemade bomb to themselves and blow it up in a crowded area at random; few groups would have the capability to attack someone hidden by the state. Infiltration allows surveillance to be used more effectively and even in a cell-based organisation allows a good amount of information to be gained.

Incidentally it is worth bearing in mind terrorists’ weakness when thinking internationally: they pose no substantial threat to us in the UK. In the Cold War there was a genuine danger of millions dying in a war that would decimate large portions of the world. Terrorists in Iraq can attack our soldiers, but they have little ability to bring the fight to us (as we can to them). The absolute worst case scenario is the closure of our borders to all Arabs, and some admittedly abhorrent curbs on the freedom of Muslims living in Britain. As a Libertarian such measures worry me, but it’s a lot more palatable than a nuclear war or the invasion of Britain by the Nazis.

It is indeed obvious however that random stop and searches in the street are an utter waste of time.