If the anti-terrorist police can't get through a firewall, and really want to look over your PC, they'll just get their pet magistrate to sign off on a search warrant, or wait until you're in the street, arrest you, and search your computer under Section 18 of PACE.
Not that it'll be much of an issue: I imagine they spooks can hack through most firewalls without too much trouble. The state's resources are vast. The only way to protect your digital privacy is to get these sweeping powers revoked.
*edit* Add "get the duty-inspector to sign-off on the search" to the Section 18 bit.