If you look at the law pertaining to the work week and opting out of the maximum, which is also in The Working Time Regulations 1998 (with my emphasis in bold):
4.—(1) Subject to regulation 5, a worker’s working time,
including overtime, in any reference period which is applicable in his
case shall not exceed an average of 48 hours for each seven days. ...
(3) Subject to paragraphs (4) and (5) and any agreement under
regulation 23(b), the reference periods which apply in the case of a
worker are ...
So if we go to 23(b):
- A collective agreement or a workforce agreement may—
(a)modify or exclude the application of regulations 6(1) to (3) and
(7), 10(1), 11(1) and (2) and 12(1), and
(b)for objective or technical reasons or reasons concerning the
organization of work, modify the application of regulation 4(3) and
(4) by the substitution, for each reference to 17 weeks, of a
different period, being a period not exceeding 52 weeks, in relation
to particular workers or groups of workers.