this may sound odd, but I have never seen a horse shiver from cold. I have seen them "shiver" , or rather tremble, from pain, like in colic, but never from cold.
I think my climate is likely near to yours, OP. it rains a lot here and winter nights are just above or just below freezing, for a good three months. The horses at "my" barn are never blanketted, except one thoroughbred.
I guess that it really depends on the horse. I always think that they must be cold, but I go to get them and they're out in the open, standing around , even in the rain, instead of seeking shelter. they must not be THAT cold.