I was in the same boat, having been overseas for a few years, so I included a letter with my entry form. Obviously it's easier if you can meet the qualifying races criteria. On the other hand the organiser has a duty to make sure he doesn't allow a complete novice and biff to take part for everyone's sake.
A couple of recent 20 mile road races might do, for example. The 3P isn't tricky for nav, but there are 3 demanding climbs, so it can easily take 30 to 40% longer than a road marathon, the race organiser needs to know you've a fair chance of finishing.
Yes, Welsh races count, of course.