The NHS Accessible Information Specification
It's a very boring title, but this is the document that tells the NHS who is and who isn't a BSL Interpreter for health settings.
The whole Specification is right HERE. I found it here. The video above is the first in a playlist of six videos that cover the whole review report (not the actual specification, which I cannot find in BSL. That's a whole other discussion....)
The Specification document, section 7.2, item 31, on page 28, says this:
I have highlighted the right bit in the Specification on this link. So, if your hospital, GP, dentist, midwife whatever provides you with an unqualified signer or anybody who does not meet these criteria, COMPLAIN. Show them this document, show them the right section, and tell them never to do it again. If they make a fuss or blame an agency, complain via the CQC here.
NHS providers have a duty to provide appropriate communication support and this document says what "appropriate" means. Booking through an agency who provides inappropriate interpreters does not discharge their duty under this standard (or in my opinion the Equality Act 2010).