The ECHR is not an EU construct, but separate from the EU, formerly EEC
The European Convention on Human Rights (ECHR) (formally the Convention for the Protection of Human Rights and Fundamental Freedoms) is an international treaty to protect human rights and fundamental freedoms in Europe. Drafted in 1950 by the then newly formed Council of Europe, the convention entered into force on 3 September 1953. All Council of Europe member states are party to the Convention and new members are expected to ratify the convention at the earliest opportunity.
Council of Europe:
The organisation is distinct from the 28-nation European Union (EU), although it is sometimes confused with it, partly because the EU has adopted the original European Flag which was created by the Council of Europe in 1955, as well as the European Anthem. No country has ever joined the EU without first belonging to the Council of Europe.
When the UK decided to BREXIT, they voted to break off from the EU - they will still be members of the Council of Europe, and subject the ECHR.