Ok, so then as you're probably aware, the legislation you are seeking to demonstrate compliance with is Part B of the Second Schedule to the Building Regulations 2006. From the point of view of the Building Regs, compliance with B1 (Means of escape in case of fire) applies to people rather than animals, therefore once the means of escape for human occupants are adequate, you're covered.
Given that some of the equine clients may have a monetary value considerably in excess of the building itself, as well as natural humanitarian concerns, it would be advisable to design in some usable means of escape for the unwell nag, such as a door wide enough to slide a horse through. I'm not sure if the equivalent of a patent slide sheet exists for horses, or if their operations are usually carried out with them on the ground or a trolley, but as was suggested above, you could speak to some veterinary hospitals dealing with these issues for advice, there's one in UCD, for example.
Again, as said above, look at having the operating room as a separate compartment, isolate areas of higher fire risk, and hopefully this will reduce the risk of such an evacuation of the operating room ever being necessary.