Net neutrality in Europe is guaranteed by the European Open Internet Regulation 2015, which was transposed into UK domestic law by the Open Internet Access (EU Regulation) Regulations 2016. Net neutrality in the UK is regulated by Ofcom.
In October 2018 government introduced the Open Internet Access (Amendment Etc) (EU Exit) Regulations 2018 as a statutory instrument to the EU Withdrawal Act 2018 to provide for continuity with net neutrality requirements throughout the Brexit process.
In June 2019 Open Rights Group published this useful primer on net neutrality, Brexit, and deep packet inspection.
Regulation (EU) 2015/2120 on open internet access provides for common rules on equal and non-discriminatory treatment of traffic in the provision of internet access services and related end-users’ rights. While these rules will, after the end of the transition period, no longer apply to the United Kingdom, they will continue to govern the provision of internet access services into the EU, no matter where the information society service provider is established.