The Department for Business, Energy and Industrial Strategy (BEIS) is responsible for trade policy and legislation, including trade defence measures such as anti-dumping and countervailing duties.
HM Revenue & Customs (HMRC) is responsible for collecting anti-dumping and countervailing duties.
The European Union (EU) is a single market. Trade defence measures are co-ordinated across the EU, rather than separate measures being applied by each individual EU country. The European Commission takes responsibility for investigating complaints and agreeing trade defence measures which are then put in place by each EU country. You can contact the European Commission Trade Defence Helpdesk on Tel 00 32 22 95 53 53.
More broadly, trade defence measures are part of the international system of world trade overseen by the World Trade Organisation (WTO). Read more about anti-dumping and subsidies.