I was International Selection Chairman for a few years. Fell races are unique to Britain so if the race is running up a track to finish on a dam high in the Alps you pick a team who might bring back medals in that sort of event. Not give your fell running mates the chance to wear a nice vest.
The Intercounties "Fell Race" was set up to provide a slightly more rugged experience than standard cross country races (ie running round a park) for runners from, say, Kent or London. So it would be stupid of the FRA to design Wasdale type courses. Having said that, races straight from the Calendar have been used, such as Settle Hills, without complaint.
Presumably selectors would apply the same criteria ie match runners to the course - if their purpose is to win medals.