I don't know that wras regs ever said you cannot mix hot and cold in a tap body - otherwise all telephone handset bath/shower taps would have been non compliant
What they do say is thou shalt not feed dead pigeon gazpacho from thy loft tank into the mains water system if we turn it off
In t'olden days the bath tap was generally fed from the loft cold tank assuring it couldn't back feed but kitchen taps often used mains cold and gravity hot. The easiest approach to guarantee compliance was the two tube non mixing approach
In your case cold coming out of a disassembled mixer tap is no indicator of non compliance. Even if correctly fitted with non return valves in accordance with regs this would still happen. Water will always find the easiest way to release it's pressure so if there's a dirty great hole in the tap body it will come out of there
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Also if you had an old style open hot water system with a header / expansion tank in the loft then the Hot and Cold were at the same pressure. There would also have been a Type AB air gap at the filler of the header tank. After this the water board aren't worried as backflow to the mains or your kitchen tap simply isn't possible.
Is that right?
The hot and cold are only at the same pressure where the cold is also supplied from the header tank.
But the kitchen Cold tap will always be fed from the incoming main, at mains pressure.
You don't want to be drinking and cooking with dead-Pidgeon-soup.
The Hot tap will be at the lower header-tank pressure.
The potential back-flow situation occurs when the mains water is off, and has no pressure, allowing for the gravity-fed pigeons to back-feed into the public water supply.
