Boiler makers set the minimum clearance for their boilers internally,building control has nothing to do with it...just get the manual for the boiler you want fitted it will tell you the clear space required.
and yes 300mm is the external distance of the flue opening where it vents the combustion gases to the outside world to the nearest window or door.... the length of a sheet of a4 paper.
and yes 300mm is the external distance of the flue opening where it vents the combustion gases to the outside world to the nearest window or door.... the length of a sheet of a4 paper.