That isn't the GDPR, although it is related.

The GDPR is UK law automatically because it is a Regulation, not a Directive (which needs to be transposed into national law).

The Data Protection Act 2018 implements the Law Enforcement Directive (as the GDPR excludes that from its scope) and a couple of minor derogations (such as changing the age of consent for children to use websites by themselves to 14).