The Romans took a pack horse - the Ariegeois from the Pyrenees up through France and into Holland. Photos I have seen were of a stocky black cob, max 14h3 type with lots of feather, always black. Maybe they influenced the black Friesians which nowadays is much bigger.
The landscape of the area of the Fells and the Pyrenees is somewhat similar.
The Romans may have brought their horses to Northern England to help build and carry supplies to Hadrians Wall. But who knows
I always took the Galloway to be an old fashioned ride and drive stocky cob who carried the Border reivers on the raiding trips. Heavy, broad backed, big butts, 6-8 inches between the front legs, coarse salt and pepper coloured hair in mane and tail, lots of bone and lots of silky feather. Working on the farm during the day, going off on a stealing livestock raid at the weekend. I reckoned my Joe was a long lost Galloway. He'd have gone raiding any day given the chance. But I'd love to hear from anyone who knows more about Galloways.
The Fells I have noticed have all been smaller - not a pony, but not a horse yet capable of carrying both adult and child.
When in the market for a horse I have looked for a large Fell or a Dales but never found one at a reasonable price.