There is no legal requirement for any electrician to have a written qualification. If you know how to do the job than that's all that is required. However in real terms you need to satisfy insurers that you are a good risk so likely they will require some proof that you know what you are doing.
I have seen many mistakes with "inspection and testing of in-service electrical equipment" which is the correct name for PAT testing in the main due to the tester being presented with some thing which needs more training than they got. One I remember was a magnet mounted drill where the drill was class II but magnet was class I and the tester had not realised and tested the whole lot as class II.
Testing the same items every time however should not present a problem. On a re-build what we worry about is a screw being too long and so being too close to some electrical part so the testing is different to standard test in that the high voltage wand is used on the screws. However touch that wand in the wrong place and you can destroy the item being tested which is why not normally done.
Class I testing is a quick look and push the button and see what the results are. However with class II there is very little tested, in the main it is the inspection which is all important. Also environment matters, I would be given a radio to test with a fig of 8 detachable lead for use in the office which I passed but same radio on factory floor failed as the lead could be detached and could fall into water. So when testing you must decide where it is going to be used.
So to nitty gritty since I expect they are all class II items question one is do they need a high voltage test? I have not worked on one so I don't have the answer. Have they got exposed metal parts? If not there is little you can test. So measuring current used with a plug in energy meter will possibly do the same as using a PAT tester. My be you need a 500 volt insulation tester but unlikely a full blown PAT tester.
In the main the PAT tester is a check list for the inspector to follow. You would need your own check list on what is tested which could be tailored to your product. i.e. cable clamp in place, fuse correct size, etc.
It may seem daft some times. I took over the work with mothers wet room when builders ran off into the Welsh hills. The LABC inspector came to visit and was not happy with my son doing the inspection and testing even though he had the correct exam passes with C&G2391. However he said I had a degree at which point it was yes go ahead, even though in real terms my degree did not really train me to do that work. I also had C&G2391 but inspector did not know that.
The C&G2377 is a two part exam one for testing and one for management, but like the C&G2382 and 2391 and 2392 they are all classed as level 3 which is the same as "A" level. But in real terms the C&G2377 is a really easy exam to pass, as is the C&G2382, but the C&G2391 has may people failing and is far harder to pass. Pre 2000 many colleges did their own courses and exams, but after 2000 the schemes wanted city & guilds exams so the internal exam vanished. Also colleges started to get league tables and allowing external candidates to sit exams they realised was dropping their score on the league tables. So may colleges stopped taking external candidates you have to complete the course to be able to sit exams. Also prices went silly in 2004 I took the three courses one after the other 2381 as it was then, 2377 and 2391 all three cost around £70. In 2008 I went back to do short course to upgrade to 2382 and price was then £160 for just the one short course and exam. And all the 2382 does is test you can read a book.