I've been saying this for over a year, the same as the guy in the video you linked, the usual apologists shouted me down, it seems there are a lot more people saying the same now, on here and in all the media, social and others.I do somewhat agree with the guy.
If you can prove intent of someone making claims of allegiance or support to terrorism, then they should be immediately detained, deported or imprisoned.
The problem for many people, is that it could create a sort of Guantanamo bay on UK soil, where you have a large number of people in 'legal limbo' who haven't been convicted of anything, locked up potentially indefinitely, for having looked at websites or attending certain dodgy mosques or teachings.
However, maybe it's gotten to the point where the public discomfort of performing such action, is outweighed by the risks of not taking real action - the harsh reality of having the blood of 8 year olds splattered across the walls of a pop concert.
Arrest and detain them all.
(I am catching up with the thread.)