The GDPR is EU legislation that has been applicable as law in EU member States (e.g. the UK) since 25 May 2018, irrespective of national legislation.
The DP Bill became law when enacted as the Data Protection Act. It explicitly brings provisions of the GDPR in to UK law and establish continuity of the GDPR in the UK post Brexit. The Act legislates in areas where the GDPR allows flexibility at national level. It also introduces legislation on processing for law enforcement purposes (in support of the EU Law Enforcement Directive) and by the intelligence services, and make provision for the Information Commissioner (the UK regulator).