According to the Snowdon race website the race is run under UK Athletics rules. I would therefore assume that the race Permit/License has been obtained from Welsh Athletics. Assuming this is the case, it will be covered by UKA insurance as are FRA Permitted races.
The UKA Fell & Hillrunning Rules (see http://www.britishathletics.org.uk/competitions/rules/) are almost identical to the current FRA Rules for Competition on which they are based. However there is a difference in the categorisation of races. In addition to Cats A, B and C there is also a Category for "International Races" . As well as international races, selection races for international races organised by national associations fall into this category.
If the Snowdon race is in this category it would be up to the organiser to specify the kit carrying requirements, they would not be compulsory even for a race that would otherwise have been AM.
When I was the FRA Fixtures Secretary, Permits provided for selection races for England Mountain Running teams were deemed to be in this category. I understand that the selectors were trying to run the trials under as near the same conditions (no compulsory kit?) as the international races themselves if possible - depending on the weather etc.