The FRA could change its constitution and declare itself the national governing body for all fell runners within the UK, but it would require a change in UKA rule 400 (5) before the FRA could actually become that body (at least as far as UKA are concerned).
I'm not sure that trail running is an appropriate precedent, it is not mentioned as a discipline of athletics in SAL articles of association. In fact, it is not mentioned as a discipline of athletics in the IAAF constitution either.