A BM Trada FD30 fire door with combined seals requires a head frame stop of a minimum depth of 12mm. Is this correct, as I cannot find test certificates on BMT doors?

If the door has BM TRADA certification it should have a circular plastic plug fitted to its hanging stile. https://www.google.com/search?q=bm+trada+plug+number&rlz=1C1GCEA_enGB1159GB1159&oq=bm+trada+plug+number&gs_lcrp=EgZjaHJvbWUyBggAEEUYOTIHCAEQIRiPAtIBCTEzODA0ajBqN6gCALACAA&sourceid=chrome&source=chrome.ob&ie=UTF-8

There will be a fir tree shape within the centre of the plug and a number will be moulded into that tree part. Quote that number to BM TRADA and they should be able to provide you with the contact details for the door manufacturer. You can then contact their technical dept for the information you need.

But, generally speaking yes. A timber-based fire door with intumescent seals will generally require a 12mm thick rebate stop at the head and jambs of the door frame.