Not according to the law it isn't.... The requirement is to be competent. (Gas Safety Installation and Use Regulations).
If one is doing gas fitting for reward, then there is a requirement for the individual or company to be a member of a class of persons designated by the HSE. Currently, the HSE requires this to be CORGI membership.
The regulations were written following celebrated events such as Ronan Point where cowboy fitters caused a major explosion, and to a reasonable extent has achieved its aims.
If you look at the HSE web site, there is a section on gas safety where there are discussions about whether or not DIY gas work is an issue and should be banned. The conclusion has consistently been that it isn't an issue and that doing so is impractical anyway.
Obviously for most people, the sensible answer is to employ a registered gas fitter. However, this is no guarantee that the outcome will be good quality work. AIUI, CORGI only inspects about 2% of installations, so unless the customer complains, there is not much supervision.
One also has to consider that a DIYer competent to do his own work has more skin in the game than a fitter in that if he screws up, he is going to run the risk of blowing himself up.
That would be a matter for the HSE, most likely.
However, you are either assuming that a DIY installation is automatically bad and a professional one is good (which is not really justifiable); or you prefer to take a conservative view and have the responsibility for a situation pushed in the direction of the fitter and his insurers. There's nothing wrong with that, except that everybody has their own views of how much or little risk they wish to take.