A serious, new plan needs to be drawn up by the powers that be. It's near on impossible to stop, but more can be done.
The problem is that the only real suggestions that are being put out there involve incredibly regressive policies like internment for suspected terrorists.
But you're right that there are things that could be done that *aren't* regressive, but require two things that are hard to come by: increased funding, and changes in attitude. On the first point, it can only be beneficial to increase the funding that is given to the police for counter-terrorism operations. We know that the majority of perpetrators of terror are known to the police for one reason or another -- though not necessarily in a terror context, as per the Westminster attacker -- and more detailed monitoring of potential suspects would at least give us the opportunity to react quickly. We've seen that the police tend to be able to round up accomplices relatively quickly; they likely already have these connections, but without proof that a suspect is about to perpetrate an act of terror, bringing them in can be counterproductive. This is where the second point comes in.
Certain communities (united by language, religion or race) tend to be somewhat insular. The reasons for this are complex, but generally boil down to a reluctance for members of the community to integrate with the wider population, and a reluctance for the wider population to accept them. We're automatically suspicious of people who look different to us, or speak a different language. We often project this suspicion in our body language and our general demeanour so it's unsurprising that integration can be hard. By being open and accepting of these communities, we foster trust and where there's trust there's more likely to be dialogue. If the police start bringing suspected terrorists and their contacts in for questioning left, right and centre, all that will happen is that the rest of that community will become distrustful of the police -- you can see how easily this happens by looking at the USA -- and so when they notice something suspicious or odd about people in their community they will be less likely to report it.
So what I'm saying is that there are things that *everyone* can do to help stop this. Put aside your prejudices about race, religion and country of origin and be welcoming and friendly toward migrants. If we show them that we're able to recognise that there's a difference between them and their subversive elements (in the same way that they presumably don't assume we're all rapists or murderers even though those elements exist in our society) then they'll be less likely to believe the propaganda that they're fed from those subversive elements. Unfortunately attitudes are hard to change, and while we have the power to do it ourselves, I'm not convinced there's the will.