Your right, Richard, the rules are the rules and should be followed. They are there to make construction safer so that we don't "all do our own thing" and everyone works to the same standard.
However in this context, I prefer a more pragmatic approach. "If I do this this way, what harm will come?"
So if the OP gets a window made (to the same spec as a company would) and fits it (to the same standard as a fitter would) then what will be the end result?
Its the same throughout the industry - if work is done to the required standard - albeit without the "correct" application or certification then so what? Does that make the work wrong or dangerous? No.
OK, this thinking can't be transposed to all building work, but for a domestic property the only person that it affects is the householder.
Yes, one has to make a conscience call .. "Is this right to not follow the rules?". But if enough people don't agree with the rules ..... break the rules ..... break the law, then the rule or the law is wrong and should be changed.
The building regs are much illconcieved for domestic work ..... be it Part P and not being able to do wiring, Part L1 and windows, Part M and needing new sockets higher than the others in the house you name it, there is little actual benefit in a lot of the current b/regs.
Should we blindly adhere to them, or should we oppose them? Perhaps a topic for another thread. lol