if the pressure's dropping there is a leak somewhere, and you say it is not coming from the pressure elief pipe that goes through the wall hehind the boiler.
If the pressure drops temporarily while you have a tap running, I should think the plate heat exchanger is perforated.
If I am right, the water from the hot taps will in that case contain some contamination from the water in the radiators, which is mixing with the tap water. You radiators will be rusting through from the inside and may rust through in a matter of weeks.
call the menders and tell them. No doubt it will need a new exchanger if the old one is perforated; but I suppose if not very old it might be a failed seal of some kind.
It might be worth seeing if you can get a fixed-price repair from BG.