A BSL/English interpreter is a person who holds nationally recognised qualifications in BSL and interpreting theory and practise. They have fluency in British Sign Language and at least one other language (usually English).
BSL is a complex and rich language with its own grammar that is independent of English. It is not a derivative of English, neither is it another form of English.
Interpreting is the action of listening to someone talking/signing, understanding what that means, and then producing an appropriately equivalent message in another language.
Interpreting is different from translation because interpreting is a ‘live’ process that takes place between speakers of different languages, such as at an international conference, or a deaf person visiting their GP; translation is less immediate and is often linked to the written and recorded form – such as translating the Harry Potter series from English into Hindi.