This issue is not as black and white as it may appear. Certainly incompetent fitting is to be strongly discouraged, but I am not convinced that the 'closed shop' is the answer. As one contributor has said, more information and reasons given is likely to be just as productive of safety. Further, how many explosions have you read about only to see that ' British Gas was working on the system at the time' ? Quite a number over the years. Compare that with the number of explosions/poisonings that you have read about for DIY installations; I've never seen any.
The other year a couple died in their Council home near to us immediately after the regulatory check and maintenance by Corgi fitters contracted to the council. I once accidentally pierced a gas supply. (It was run embedded in the plaster in a most unexpected position). I was attempting to fit a picture hook - Bovis Homes, incidentally. I called British Gas. The fitter saunterd in smoking a cigarette. I queried the safety of this to be told 'You've turned off at the meter, so should be OK'. He then got out a blow lamp lit up and dobbed some solder over the hole I had inadvertently made. Well we survived, but it seemed a bit gung ho to me and may account for some of the other BG incidents.
More recently I had occasion to use a Corgi fitter for our campervan LPG installation. He made an absolute pig's ear of the pipework and we had a couple of leaks which he tried to convince me was my imagination. He reluctantly produced his instrument, a handwash soap bottle with some domestic solution, and then found that there were indeed leaks. These he tackled with a pair of Stilsons on my poor 8mm fittings, not making any attempt even to stop the fitting from rotating under this attack. I diplomatically persuaded him to use a spanner on the backnut and check the olives which he reluctantly did. I am not suggesting that all gas fitters are like the examples I've given, but it does illustrate that having a certificate is not the complete solution and that information and properly trained people is better. He incidentally, I subsequently learned, had just been made redundant in Leeds because his firm had employed instead several fitters (calibre unknown) from Poland as these were cheaper.