The EU General Data Protection Regulation (EU) 2016/679 of the European Parliament became law in 2018. At the time, the UK was one of 28 EU countries that implemented the law as the data protection regulation for their country.
With Brexit the UK separated from the EU and was therefore responsible for implementing its own data protection law. The UK implemented the “Data Protection Act 2018 (DPA 2018)” which is similar in context to the EU GDPR. The UK has tight data protection laws, same as the EU. In doing so the EU adequacy decisions apply.