I seem to come across a lot of this sort of thing (like everyone else here I suspect). Sadly it doesn't seem to make a lot of difference whether they are part of a scheme or not as I see some atrocious work supposedly carried out by competent persons. Personally I don't think part P works at all and is just another pointless money ground. It certainly hasn't done much to drive poor workmanship out of domestic electrical work!
Most recent job was on a house that had been rewired around 5 years ago, homeowner had building control cert but hadn't been given an electrical installation certificate. Considering the quality of workmanship I wasn't surprised and they were supposedly a member of one of the more reputable schemes!
Considering it was modest 3 bedroom house that had been extended to include a fourth bedroom they had been left with 5 socket circuits and 4 lighting circuits, a new cooker cable that had been damaged when it was pulled in, hidden non maintenance free junction boxes all over the place, unsupported cables, undersized main earth and more. The image below shows the state that the service head etc. was left in.
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