I don't know what the regulations are in the UK, but in many European countries, building a house isn't just about knowing how to build a house, it's also about knowing all the DIN requirements. These are necessary for insurance to pay out if something happens to your house. Let's take a simple example: You're wiring your house. Because of a mistake, the house burns down.

Insurance: “Okay sir, who did the electrical wiring? you: “Me”. Insurance: “Are you a professional? you: “No" Insurance: “Have you had your work certified by a third party?" you: “Do my buddies count?” insurance: "Have a good day sir"

There may never be a problem and you may have figured out everything that's required to get your butt covered (good for you in this case), but the fact is that a lot of people don't know about these things, do their own thing and get royally screwed if there's a problem and, God forbid, someone gets hurt.