It starts off black & white...
- Normal maintenance is permitted even in special locations
- Adding mechanical protection is permitted
So no problem with...
- Replacing existing SWA across a garden
- Replacing socket or light fitting anywhere (age, aesthetics)
- Cable replacement (heat, chemical, mechanical)
- Backbox replacement (rust, damaged screws)
- Oval/Capping fitment during redecoration (mechanical protection)
However it gets a bit grey...
- Sinking surface socket in same position - protection, maintenance?
- Changing 2G to 1G+1G - maintenance if two 13A loads (2G limit is
19.5A)?
- Like for like - by powered (wireless/mains)?, function (switch)?, type (switch/dimmer)?, brand?, spec?
The legal reading is whether the change constitutes an added socket or light, if not then it is based on function & maintenance. The legal reality is watching the prosecution argue the toss between an MK Echo light switch, MK Dimmer light switch and MK light switch and the judge asking "so... you have not added a socket or light... the function remains the same... or I just can't tell shit from shinola?".
Supposedly Part P fees are to become "better scaled to the work" which would end the "SI2004 applies" myth (to the extent that some neighbours have been ostracised and even police called as "illegal"). Would be bloody funny watching Crimewatch discussing how it was "dreadful" someone fitted a socket in place of a SFCU having changed from a built-in fridge to a slide-in fridge - whlst another Baby P #2 headline scrolls across the screen. Yes I am being cynical, because socialism & protectionism are a disease not a treatment.
Re OP, the other issue about voluntary is w.r.t. *your* insurance. Not just whether the organisation's insurers impose any restrictions (they may not, but may go after anyone causing them to incur any costs). With any work a good rule is photo-before, photo-after - Part P1 imposes the legal requirement which is a lot more practical than what John Prescotts stakeholder bodies wrote.