Just to pitch in, I agree with Mike Proctor on the 3 fingers rule - which the clarification that I understood that the 3 fingers gap was between your nose tip and the nearest point on your trigger hand along the comb (i.e. part of the thumb)
I also agree that in my limited experience the UK fits tend to slightly longer LOP, slightly less bend (down) and more cast compared to American. There is interesting discussion of the differences in style and how they came about particularly in "Shotgun Shooting, Techniques and Technology" by John Brindle (no connections!)
