I once asked a WPD intake team who were drawing in a new line for a remote new-build to put in an isolator for me. Was told that they couldn't do it as technically because it was downstream of the intake it came under Part P - and they weren't part of a scheme!!
That was a good few years ago now, hopefully things have moved on somewhat....
Although I had a bit of a headscratcher in London back in lockdown, conversion of an old warehouse into a BMO - I was installing the initial BNO infrastructure. Trouble was, the developer wanted to move into one unit of six before the rest were even remotely finished, basically living on a building site. Talked directly to the UKPN engineer and when we came to move the 200/3 intake position I installed an adjustable ELCB before the run out to the Ryefield and meters. We mutually agreed it fell right off the charts in terms of normal regs and proceedures for both of us but was the right engineering thing to do.