Mixed gas is recognised, but BSAC does not (as yet) issue it's own qualifications. Yet. One NI in our club has been known to dive a Draeger, and another has done an Inspiration course for the fun of it.
I'm sure one of the rebreather guys will be along in a moment to give us the proper line, I'm afraid I don't know chapter and verse as I'm still saving.
In the Diver article Chris Boardman made the point that he was a BSAC Sports Diver as his CCR and trimix quals did not cross over to Dive Leader, and IMHOTEP, I don't think they should as it's like comparing apples and oranges.
Laters,
Janos