The Bobbies (gawd, I might be in trouble with that word

) may have weak evidence (suspicions) that Mohammed Doe is a terrorist. But not enough to file charges. So the compromise position is to hold him and seek better evidence. Or find out he is perfectly innocent and let him go.
Well, I think that is exactly what the supposed purpose is.
But many people think that locking people up for long periods while you try to find something you can charge them with is a Bad Thing, and that you should have more reason than a tingling spidey sense to act on.
This is a matter of principle, but also stands against the background of a number of rather high profile cases (
e.g.) where the police have lost public confidence by being certain that people committed a particular crime but having no evidence to support it, and so invented some evidence to ensure conviction. Consequently, confidence in police suspicions is not (or should not be) high enough to allow lengthy detention of citizens based purely on that suspicion. If they have
more than suspicion, i.e. some actual evidence, then fair enough.
There is a caveat to that, which is that last time I checked wiretap evidence is not admissible in court. So it is perhaps arguable that an intercepted phone call could give a very good indication that an attack is imminent, causing the immediate rounding up of the people involved in the call. But it might take time to get some non-wiretap evidence to make it a charging matter.
Of course the solution here is not to lock people up for weeks, but to make wiretap evidence admissible. God knows why it isn't already (I believe there are moves afoot to make it so).
Although the posters from the UK who've responded to this thread are universally against this change, in the interests of balance I should point out that according to the radio this morning most of the opinion polls taken by the press have indicated that a majority of people support the change. And not all of them on the grounds that it's only brown people who will be affected - I've seen quotes along the lines of "If we lock up a few hoodies [i.e. young people hanging around in groups] that will bring crime down too"