Which is why many houses now have GPOs (power points) in the ceiling space, and the light fittings are appliances plugged into the GPO, rather than permanently installed. Domestically, it is legal to work on things that are not permanently wired in, the main constraint being that you should follow good practice, so you don't kill someone.
I'm surprised that a keen DIYer hasn't tried using a 3-phase 32A plug to plug their house into the mains, and claim the entire house is an appliance.
Unlikely :-) That final comment was tongue in cheek. It would make an interesting argument, but I suspect the house wiring would still be viewed as "fixed", despite the plug.
My gut feeling is that the law doesn't really achieve much, past a level of discouragement via FUD, since it's pretty well unenforceable. Electricians are supposed to write out "compliance certificates" for any work done, but I gather an awful lot don't. Even then, unless the work impacts the external connection, the only copy of the certificate resides with the home owner. I imagine an awful lot of people file it under the sofa, or in the garbage bin. As far as I know, there is no central record of a house's wiring configuration. Practically, the only way of telling that a job wasn't done by an electrician is that the job is really bad (and dangerous), or of a standard that significantly exceeds that achieved by a typical electrician.