thing is it wont help with crime or terrorism (ok some super thick person might get caught but now they know the government are doing it they take steps to bypass it)
what it will help with is when china (or one of the other new powers) come knocking on the government door and say " we just place a multi million pound deal with you, can you give us a list who those Chinese dissidents where talking to" and the goverement can pust a few buttons for the info and no one need to know
and anyway it not as if the government is going to use laws that are for anti terrorist for some other reason, as if the pm would use laws made to seize assets of a terrorist, against a bank or a country because his mates might loose some money, opps it all ready has