Perhaps you would share some of the other ways that the DIY'er can satisfy the requirements Part P.
Well, in the context of this thread, I think I already have done - by appealing to a common sense interpretation of the law - don't forget that it is judges and juries who make these calls, not BS7671-indoctrinated people ...
... in the sort of example we were discussing, imagine that there is an existing ring final circuit, about 30m in total length with 15 socket outlets, the cable being buried in walls, but not 50mm deep, which has no RCD protection. Even BS7671 accepts that this is 'OK' and that there is no need to rush out and upgrade it with an RCD so that it met what would be the requirements of BS7671:2008 Amd 1 (2011) for a new circuit.
A DIYer breaks the ring and inserts a 16th socket, entailing the addition of about 12" of new cable (again buried <50mm deep, in a 'safe zone'), the actual work all being undertaken 'competently'.
Part P of the Building Regs requires that 'reasonable provision' be made in the design and installation of the new socket to minimise the risk of fire or injury to persons. The prior ciruit was not considered unsafe, so I would hope a court would accept that 'reasonable provisions' had been taken if the additional socket and foot of cable had been incorporated in exactly the same manner as the remaining 15 sockets and 30m of cable, the actual wiring work having been undertaken competently.
Of course, I can't second guess the decision of courts, and therefore could be wrong - but what do you think? In my view, by wording Part P so loosely, the scene has been set for courts to apply common sense - and I hope they would. They may, of course. 'fail' the DIYer for not having tested the circuit adequately after the work, but that is a whole different discussion!
Kind Regards, John.