I'm also in a position of retraining and currently weighing up my options.
If (IF) you are only looking at doing domestic stuff, as I am (based on very little industry in my area), then you can do your 18th Edition (C&G2382-18:A2) and your Initial verification/testing/inspection (C&G2391-52) as
@swaRRR mentioned up-front (these will be required for just about anything electrical) and you could do the C&G5393 - DOMESTIC NVQ3. Within this 5393, I have found that there is a module for 18th edition, and a module for Inspection and testing, so what this option may offer you, is a way to START getting some paperwork, and then cut the modules out of the 5393 course. They allow what is known as RPL - Recognised Prior Learning, where if you have covered all of the material in a module, then you can bypass it by applying your already earned credentials to it.
Here's the downside though - 5393 hasn't officially been given the go-live yet! However, there have been updates to the C&G 5393 page within the last 8 days, so there is 'some' movement on it. 5393 also has no pre-requisites for 2365 L2/L3 diplomas, so you can start directly on the NVQ3 DOMESTIC.
If you are already in plumbing/gas, then you are unlikely short of work anyway, so learning in between jobs on the 18th edition and 2392-52 will be a good use of your time until C&G pull their thumb out of their peach!
This of course, is dependant on you only wanting to do domestic stuff ONLY!
If you want to do commercial too, then scrap everything I said about 5393 and do as many people have already told you here and get a job doing the donkey-work onsite and get an employer to put you through the Apprenticeship IF YOU HAVE THE TIME!