Imagine the scenario: Mr DIY has been living in his house for x number or years and tinkered and DIY'd his way through the house. Everything 'works' and there's no obvious sign of amateur wiring. He's decided to sell his house and to help it sell, he's popped down to a big shed and bought himself a consumer unit. Somehow he's blundered his way through it but inadvertently he's turned the upstairs socket RFC & downstairs socket RFC into one huge RFC by picking up the wrong wires in the consumer unit. Well, they all looked the same didn't they? It's not like he would've r1&r2'd or Ze'd etc. Also, where he's moved light switches about, he's borrowed neutrals from nearby sockets. It all works, sure it does. An inexperienced, newly qualified sparky, on his first proper job, may not pick up these massive errors because, when you're on your own for the first time, things like that happen. In addition to all of that, if it's for an EIC, how do you know what reference method? what grouping factor? etc etc......... For all you know, there's a 10mm cable feeding a shower that runs under 200mm of loft insulation for most of its length. Seriously, if someone else did it, and you didn't see it go in, you don't really know anything about that installation other that what you can see at the ends of the cable. That's not enough to put your name to it as safe/correct/within regs. For all you know, they've sawn through half a joist to pull a cable through. When that floor collapses, who do you think they'll come after? You don't want to be a fall guy for anybody.
