The most likely breed, is whatever breeds that are commonest in that area of England.
The mealy muzzle is common in dozens of breeds of horses and ponies, I have had a Thoroughbred and a Dutch Warmblood with the same markings. I would not go by the color. And in fact Exmoors are usually much more distinctively marked than this one.
With the blanket on, the conformation and even the proportions are hard to see.
I think that depending on the region, there are usually a lot of various grade(unregistered, crossbred) ponies in England, various crosses of Welsh and native breeds.
I think all of the native pony breeds are fairly well represented al across England, either as purebreds or in crosses, but a native person can give a better idea of that.
It's a beautiful little pony, no matter what breed it is.