Whatever MI's say, they can't take precedence over building regs (in this case Part J Combustion appliances). It's no great surprise that MI's nearly always just reprint the relevant pages of the regs.
The pages below are taken from the latest issue of Part J. Note the horizontal distance of the flue opening from an opening window of 300mm - this is edge of opening to edge of flue.
See also Note 1:
"An opening here means an openable element such as a window, or a fixed opening such as an air vent. However, in addition, the outlet should not be any nearer than 150mm (fanned draught) or 300mm (natural draught) to an opening in the building fabric formed for the purpose of accommodating a built in element, such as a window frame"
Whatever the MI's say, they can't be less stringent than the building regs and as an installer making holes in buildings for flues, you must take account of the building regs
in addition to any MI's. The regs are free to download - it's worth taking a look:
Planning Portal - Part J (Heat producing appliances)
View attachment 4961View attachment 4962