Meindl Borneo and Lowa Munro are two good quality leather outer / leather lining options. (I've had a pair of Borneos for about 8 years now - like you, I move into them from trail shoes for winter walking).
But they don't quite hit your £100 criteria. I've seen the Borneos for about £150 and the Munros for £130. I suppose you might get lucky and find them on a better offer somewhere.