The British government - either party - just doesn't seem to "get it" with respect to civil liberties. Poor logic comes up time and time again, the public don't really care much unless they read The Guardian. It makes me glad I left to a liberal cheese-eating European country ;)
At university a law student defended (regurgitated a professor's opinion) our lack of constitution by saying that all the important parts end up enshrined in law anyway and that the constitution's lack of flexibility causes problems as society/technology changes. I think there's more to it than that - the mindset that accompanies having a constitution (particularly in the general population). At least in the US encroachment tends to happen with extreme events and is duly challenged as unconstitutional.