The Brits have a civilian category of instructor which allows PPLs to instruct. It’s really intended for flying clubs. They are not permitted to send a student solo. At club level I am not even sure that they have to be supervised by a higher grade. Someone more familiar with that system could comment?
Because there is no military equivalent, no RAF rating is recognised by CAA for this, even if a QFI elects to only hold a civilian PPL
A RAF QFI who converts to a CPL or ATPL can also get an instructor rating, subject to flight test. Privileges of that rating are dependent upon what said QFI did while in the service; e.g. to be able to send students solo, must have already done certain minimum number.
CASA’s approach is very similar, though in the past may have been inconsistent. No surprises there!