In my opinion I do not think it would be a good commercial venture.
I do not think that there will be a sustainable client base.
All new systems are to be inspected by a bulgarian inspector and a passport issued, for bulgarian fitters this is not a problem and passports are issued only after a cursory inspection, I do not believe this would happen with a new brit fitter.
If you are prepared to go through all the red tape (health and safety, fire regs, insurance etc) in setting up premisses and anything else the locals can throw at you and recoup your set up costs while competing with bulgarians who do not have the same problems and earn a living I wish you good luck.
I had a duel fuel system fitted on a zapheria for 500 leva. The vehicle has been back and forth to the uk three times and is in daily use I have had no problems with it. For peace of mind I had it checked out on the first visit to the uk by both the main agent and a lpg fitter both said it was safe.