Terrorists aren't caught by invading civil liberties. They're caught by diligence, police work, and cooperation between MI5 and Scotland Yard.
Any time a law is made, the worst case interpretation of that law needs to be considered. Even if the current government won't misuse the law, someone further down the road, will. The Terrorism Act is an example of a law that was written so broadly that it can be interpreted to mean that police can do almost anything they want, if they claim it's for 'National Security.' The truth is, the Terrorism Act is frequently cited by judges, the European Commission on Human Rights, and civil liberties groups as one of the most egregious violations of civil liberties and privacy rights ever written in the UK.
It's a terrible law with no moral or ethical justification.
"Those who would give up Essential Liberty to purchase a little Temporary Safety deserve neither Liberty nor Safety"