Dave H. wrote:

I'd go for a downward slope through the wall, then an elbow pointing the
pipe downwards on the outside - I have faith in Gravity To seal the hole
you could try glass-fibre rope (wrapped around the pipe and packed into the
hole) from the Proper plumbers' merchant / stove shop to avoid draughts, and
a piece of suitably-bent ally sheet over the elbow to deflect the rain?

I'd suggest glass rope for packing and high temperature silicone sealant
to finish off inside and out. The tube really does need to slope down to
prevent rust and all joints inside need to be gas-tight to prevent
carbon monoxide build up. Ventilation also required as with a boiler.