S.P.S.
Gas Engineer
The flue is'ant sealed around as the products of combustion can get into the loft can't be passable surely
That isn’t an issue per say, the plume is directed past any over hangs/protrusions that could effect the clearance of flue products. You could quite acceptably have a vented soffit board running right the way across. Most eaves are ventilated in some manner.
Stu.