Well if you do not mind traveling down south or maybe they can travel up the country, Phil Short, Simon Ross Deveau, Martin Robson. Have a look on the IANTD uk website, you can get a listing from there.
As for TDI course I am not really sure, but I do know there are loadsa trimix instructors avalible through TDI but there are only a select few IANTD ones in the UK, the course requirements to get to IANTD Trimix Instructor are not that easy, not that I am saying TDI is but, the number of instructors avalible I think has something to say for itself
but not having done either course with either training agency, I would be honest and say that it really does depend on the instructor and what you want out of the course
one little pointer I always mention on a comparison with IANTD and TDI, with IANTD you are qualified to dive any depth any gas any configuration with TDI (if I recall correctly) you are limited to 70-90
?
not sure
beck