If a production wishes to engage a UK-based 1st AD, Bectu can issue a waiver letter that releases the production from the contractual obligation to engage a DGA 1st AD. The DGA recognises Bectu as the appropriate regulatory agency under their rules. This will allow the production to avoid any of the penalty payments that would normally be due for employing a non-DGA 1st AD.
Bectu have produced this guide.