It is with very limited exceptions illegal to require your staff to opt out, even in the UK, and the same restrictions applies to taking punitive actions against staff that chooses to opt back in.

EDIT: For my part I've never worked for anyone asking for an opt-out, and I'd never sign an opt-out, and I would sue if I suspected an employer punished me for not signing an opt-out.

EDIT2: Here's the UK government advice on the opt-out [1]. Note: "Your employer can ask you to opt out, but you can’t be sacked or treated unfairly for refusing to do so. You can opt out for a certain period or indefinitely. It must be voluntary and in writing." Additionally there are exceptions for a very small set of types of workers [2].

[1] https://www.gov.uk/maximum-weekly-working-hours/weekly-maxim...

[2] https://www.gov.uk/maximum-weekly-working-hours/overview