The law requires that the driver must have a prescribed rearward view of their vehicle and trailer. It does not prescribe that that view must be provided by extension mirrors. However when towing with most combinations of car and caravan, the standard mirrors or rear view system cannot provide the prescribed view, so extension mirrors are usually (but not always) necessary. The law in this matter does not differentiate between the purpose of the trailer.
I am however highly upset at some of the views expressed in this thread. they are bigoted and inflammatory and directed a perfectly legitimate type of road user. Why this was started and allowed to be perpetuated in a caravanning forum topic area for "Caravanning" I do not know.
Even if it was meant to be a humorous topic, it misses that mark by a country mile, and I think it should be removed completely. It is totally inappropriate.
For the record I do not tow horse boxes, I am just appalled at the hypocrisy shown but forum members.