UKA / England AA now operates an individual membership scheme, so when you enter road / trail races you're either a member of an affiliated club who have registered you with England Athletics and you get a reduced entry fee, or you pay the unattached levy. FRA don't pay the individual membership fee to EA, so you still need to pay the unattached levy. You could belong to a club and not pay the EA membership fee, which would be fine if you just did fell races, but you'd have to pay the unattached levy in other races.
Or it could be an ARC permited race who I doubt recognise FRA as an affiliated club anyway.