I don't think Building Regs specify a minimum height for a flue outlet
above ground level, only a minimum height above the roof of the
building it's in - and your shed may fall outside the scope of the
regs anyway.

But - and it's a big but - the taller surrounding buildings nearby may
cause odd air pressure variations depending on the direction and
strength of the wind, possibly resulting in chimney downdraughts.

Practically, you'll have to light it and see. But the higher you can
get that flue outlet, the better.