Well, in this case the proposed act will basically supersede the Human Rights Act, by excluding authorities from respecting its article 8 ("Right to Privacy"), under which they've been repeatedly challenged (and defeated) since Snowden's revelations. Which is really a change in law, not a clarification.
So yeah, what they really need is a change, because current law is very clear that what they do is illegal.
No, it doesn't (and can't) supercede the HRA; there is actually no mechanism in UK law for doing that other than explicit repeal (Factortame principle).
So yeah, what they really need is a change, because current law is very clear that what they do is illegal.