Re: Anybody who knows anything about plumbing ...
In the UK, in order to prevent the possibility of CH water (which is probably full of anti-corrosion chemicals) flowing back into the mains, the filling loop must be fitted with a double check-valve (non-return valve). In fact the regulations used to say that the loop must be completely disconnected when not in use. I think this has changed these days, as some boilers are permanently connected to the mains and can indeed auto-top-up.
In the case of a gravity-fed system, there must be an air gap between the filling valve outlet in the header tank and the high water level.
While you can't initially pump up the CH circuit to more pressure than is in the mains, mains pressure does fluctuate and of course the action of heating the CH water and indeed pumping it around will raise its pressure, so it's possible that the water in the CH circuit can be at higher pressure than the mains.