Making a bomb that goes off when (and only when) you want it to is not easy, and there is a very limited number people in the country who know how to do it. The authorities know who most, if not all, of these people are. If you decided that you wanted to make one there's every chance that you'd end up contacting one of these people, whose communications are monitored. Bingo, you're on the radar.
The chance of being caught up in a particular attack at a particular location remains very very low, so we shouldn't worry unduly. Be alert, report anything untoward but don't over-react.
So all these muslim males wandering in from Europe dont know how to make bombs mmmmmm i bet a few do???