UKA represents all athletes in England, Scotland, Wales and Northern Ireland.
If the FRA is to represent only England then it would have to affiliate to England Athletics.
There is nothing within the constitutions of UKA or EA that would prevent the FRA representing the interests of all fell runners (similar to the Trail Running Association).
Is is possible that Scottish Athletics or Welsh Athletics may have wanted to control athletes in all disciplines and this is the real reason for the split.