THERE IS NO SUCH THING AS A "PART P" ELECTRICIAN
THERE IS NO SUCH THING AS "PART P" QUALIFICATIONS
There is notification of notifiable works according to the Building Regulations, and the exemptions to prior notification fi the work is done by a member of an approved scam... err I mean scheme. It's unfortunate that Part P (which is only one paragraph and basically says that works must be safe) was introduced at the same time (2005 BRs) as notification requirements were extended to some electrical works.
IMO there is little point asking the LABC dept about something as simple as that - it gives them the opportunity to say no and have little chance of having to justify it. So they'll take the easy route and just say no to anything that doesn't fit their narrative. LABCs are also well known for making up their own requirements (like needing an extractor fan in a room that's got an opening window).
If presented with a professional looking certificate for the works (especially assuming the person has appropriate trade insurance), then they should accept that.
I did ask LABC at my last house. I was doing some minor building works so got them to give their advice up front, and to be fair, they were very helpful - even told me what size concrete lintel to use that would make them happy. A friend has been doing some major works over the last couple of years, and he's found them very helpful as well. As for the CU replacement I had planned, he asked if I'd be getting someone to test it - I replied that I'd provide a set of test results and he seemed to be happy with that.
The key thing is that if provided with reasonable looking paperwork then their back is covered. Present them with no certificates, os some scrawl on the back of a smoke packet and they are going to be "sceptical".
For a board change, just do the work and issue a professional looking EIC. The homeowner is responsible for prior notification to LABC, and paying the fees. Technically, the "lying when filling in the TA6 form" can be got around by ticking the box for "yes, there have been electrical works since 2005", adding a note that no there are no LABC completion certificates - but here's the EIC. There's still the issue of not complying with BRs, that's time limited anyway (is it 4 years after which the LA can't do anything ?) if the householder isn't planning on selling soon.
As an aside, my LA (South Lakeland) charges ÂŁ180 fees for a CU change.