Technical Booklet L Combustion Appliances and Fuel Storage Systems·Page 41·3.11
General requirement for flue outlets
Flue outlets must be above the roof where combustion products can discharge freely without presenting a fire hazard in any wind conditions.
The outlet from a flue should be above the roof of the building in a position where the products of combustion can discharge freely and will not present a fire hazard, whatever the wind conditions.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet L - Combustion Appliances and Fuel Storage Systems.pdf