You don't need to monitor everyone. Just those that get flagged. Then they are easier to monitor
A system set up specifically to monitor everyone (which is what the current government's publically declared goal is) will never be used only to monitor people who have been flagged. The whole point of it is to monitor everyone (or as many people as is possible with existing technology).
When a system is designed to monitor and restrict everyone in a country, that's what it's for. The plans are extremely badly suited to anti-terrorism work and may well be counter-productive by generating far too much irrelevant material. The plans are extremely well suited to internal oppression, though. It might be that the current leaders are so ignorant, stupid and incompetent that they don't know that and can't understand it even when security experts tell them...but I doubt it.
Even before the recent slide into ever greater authoritarianism, "anti-terrorism" monitoring was being used by local councils to find people who put their bins out before 1800 on the day before collection. I'm not sure you could even call it function creep when the system is set up to monitor and control as much as possible about everyone (apart from MPs) and "anti-terrorism" is just a useful lie.