Well, that of course is premised on the words "the UK undertakes to be a party to the ECHR" or equivalent, expressly or impliedly, appearing. 5(b), the important bit, doesn't say that. Nor does it say what GP claimed it does in the first post, or anything like it.
I’m going to throw in a little googly here. The GFA is both an international treaty and an expression of the welter of still extant side-deals with paramilitaries. Binding the UK to the ECHR is *highly* important to some of them. Unilaterally breaching that obligation would be seen as perfidy.