Mass round ups? Who exactly would you round up? this bloke wasn't on their terrorist suspect lists, he was just known to police for other matters.
Well, oine suspects that the police and security services know of many individuals they are pretty sure are radical, many mosques where such radicalism is propagated and so on, but lack hard evidence to address these matters within the bounds of the law; it is feasible that, at some point, the government will allow them to, shall we say, stretch the limits of the law? See 'Big Boys Rules', the story of the SAS in Northern Ireland, for example; or, indeed, the SAS in Gibraltar.
Big difference between a handful of Paddies and 20 million Allans spread throughout Europe. Internment camps dealing with such large numbers would quickly become concentration camps (in the true sense of their original meaning of a facility to contain large numbers of people).
Think how bloody messy it would become.
I really do feel for our French comrades. They are having a terrible time and seem completely powerless to stop the Allans striking at will. I wonder how long it will be before the Front National comes into power, as I really do think that it's the path we're heading down.
We should be thankful that we're surrounded by sea and not part of the Shengen zone. Our security forces appear to be doing ever such an outstanding job as well. I really hope that our decision to leave the EU does not impact their ability/willingness to share intelligence as it's clearly necessary in these uncertain times.