Re: Anybody who knows anything about plumbing ...
... knows you don't plumb two water sources together. Ever.
Erm - AFAIAA most CH boilers in the UK have a means of temporarily coupling the CH circuit directly to the mains water supply in order to fill or top-up the CH system. In my recently installed boiler it is achieved by opening two ball-valves using a screwdriver. In my case I do so once or twice during Winter when the CH pressure falls below 0.8 bar (probably due to small leakages in the CH circuit). This is explicitly detailed in the instruction manual for the boiler - it is not an unapproved arrangent put in by a cowboy plumber.
The CH circuit has chemicals in it to prevent scaling and freezing, so we would definitely not want it to leak back into the mains water supply. AFAICS the only thing that prevents this from happening when both ball valves are open is the assumption that the mains water pressure will always be higher than the CH circuit pressure. While there might be a non-return valve somewhere inside the boiler to prevent flow from CH to mains (I don't know), a non-return valve (if it exists) would not negate the fact that the two water sources are plumbed together.
So I am not sure that your observation is correct.