Just checked on my local BCO site. Medway.gov.uk

You can do the work and arrange for a test by a competent registered
person. But not surprinsingly they warn you that you will have
difficult finding anyone prepared to do this.

You can do the work yourself, but you must submit full plans. You then
pay an additional fee (unspecified) on top of the standard BC fee and
they will arrange for someone to do the test.

This is a rip off.

What do other councils say?