SGH
As Gary says it would take some dice throwing of monstrous proportions to wipe out a deployed gun battery before the battery could inflict at least some damage on the Brits.
Did your guns throw double one and go low on ammo?
That would help but still you would have needed to keep rolling double ones to the Brits double sixes.
I suspect you didnt apply the half casualties modifier when the Brits were firing at long range.
A poor dice throw for a 3 gun horse battery on morale when suffering 10% casualties could I suppose cause them to be "swept away" quite quickly