Some may do but most don't. I think (and it is only a thought) Petplan may have done at one stage but then their premiums are quite substantial anyway so probably only reduced it to a 'normal' amount!
Never come across one yet that does. For a while i kept my horse on an Army base, behind the razor wire with armed guards on the gates & regular armed dog patrols. Still didn't get anything off the insurance even though it would have been extremely hard to steal her!!!!!
Some companies used to when freezemarking was a new idea (think there may have been an "incentive" between the freezemarking company & insurers) but as numbers marked got higher the idea just disappeared. Don't know of any insurer that discounts for this now (or microchipping).