Isn't the whole point of the building regs that what you build must conform to a minimum standard?
If you refuse to comply with the building regs, they hold powers to rectify whatever you did and send you the bill.
If you refuse to pay that bill, you can end up in prison.
So, yes, I'd say that Part P holds legal status. If I refuse to acknowledge it, it is proven that I refuse to acknowledge it and I refuse to rectify my deviations from it, then refuse to pay for those rectifications, then I could go to prison.
But, you'd have to be pretty silly to go to prison just because you want an electric shower in 1.0mm flex, on a 1 million amp rewirable fuse, in their garden, within reach of a solid copper bathtub full of salt water. Some daft person will, but most would pay the cost to get it certified or remedied way before a magistrate became involved.
However, if you are unlikely to sell your house anytime soon, are electrically competent (but not certified), and plan to meet the regs anyway (in case the house burns down and the insurers find sub-standard wiring in new colours) then I doubt it will really affect you.