You are confusing competent (we all know what that means) and "competent".
In the eyes of the law you would have to demonstrate "competency", which
"they" generally take to mean being CORGI registered. The onus would be on
you, if not a CORGI, to prove you were "competent" (even if competent and
IF
anyone asked of course!), which would involve showing you had lots of
experience, satisfied customers, years in trade etc.,

I don't think they would be quite so strict.

The fact that the law only defines "competent" to mean "CORGI" for paid work
means that you can infer that the intent was not to define "competent" to
mean "CORGI" for own house DIY work. Otherwise, they would have simply
defined "competent" to mean "CORGI" for all work.

That is not to say that they would expect the highest levels of workmanship
and skill etc. You'd have to do the job just as well as a registered fitter.
No skimping on the leak testings, or not wrapping buried pipes or leaving
flux all over the place.

I regard myself as competent at most things, but I still wouldn't do gas
work myself (beyond connecting a bayonet), as I don't think I know about it
in enough detail.

Christian.