An English breech (note spelling) in muzzle loaders is a breech in which the touch hole or nipple seat is contained in the breech plug rather than in the barrel. The threads are ahead of the breech. I am not sure it is that much stronger, but it is easier to correct a problem with the nipple seat or touch hole, since the breech plug unscrews and can be replaced without replacing the barrel. Here is a picture:
