The Public Health (Control of Disease) Act 1984 empowers ministers to make regulations in regard to the current situation.
The Health Protection (Coronavirus, Restrictions) (England) Regulations 2020 [as amended], or equivalent regulations in other parts of the UK, provide RULES as to what people may or may not do and empower the police to issue Fixed Penalty Notices when those rules are broken. For example, people attending a mass gathering (such as a 'rave') will be breaking the rules and can be issued with FPNs.
However GUIDANCE is laid on top of those rules. The police have NO powers to enforce such guidance. For example, it's only 'guidance', and not 'law', that people should maintain social distancing. If the police see two people, from different households, hugging and kissing, they can offer advice to them but they can't issue an FPN to either of them.