[derekgas]

Yours would just be 'not to current standards' Neil, and I'm fairly confident you know what that means, if you are fitting a double check valve in your own home, you can put it where you like, the idea is to stop water siphoning back out of hose pipes etc, into the drinking water.
As for not interrupting a mains flow to an outlet, that is what you would be doing if you put one of the screw in contraptions before the tap, in fact, put it anywhere in the cold water pipe, and it will restrict the flow to something, it is unnavoidable.