When the govt introduced the separate licence for large goods vehicles, (I think it was the 1968 regs) they said that if a driver was able to prove that they were regularly driving HGVs they would be issued with an HGV licence appropriate to that vehicle. BUT. To achieve that the Company one worked for, usually the MD or senior transport manager, had to certify in writing that you had been so employed regularly for I think 12 or 18 months. This first issue dealt with the potential problem of thousands of truckers being without a licence. In the main it was successful as responsible Companies would not certify non users. However, many years ago I came across a guy who used to sweep up the yard, literally, and he obtained a licence this way.
Whilst grandfathers rights tdo eature in DVLA doings it does not apply in the cas e of this question.
However, if you look at the rules covering the certification of prof competance for HGV and PCV drivers there is a grandfathers grace. That is to say if you currently hold a full HGV licence, even though there is a requirement to achieve the CPC, there is a five year gap where you are exempted. So if you are thirty yrs old - get it. If like me you are nearly sixty, it probably is not worth it.