Personally i wouldn’t bother traveling for short events and that’s reflected in my suggestions. 🙂
The Bearbones200 (200km) is in October. Unsupported ITT mass start type event. Generally some of the best terrain that mid wales has to offer, although it will be a new route this year and I think it might go more into North Wales. Get ready for all sorts though. There was at least 15 river crossings last year, a wack of hike-a-bike and plenty of route on the map but nothing on the ground type navigation challenges. Times ranged from 13ish hours to 28ish for those that slept out over night.
Highland trail race (560miles) if you can hack 5-8 days in the wilderness. Last year and this years line up contains international riders. That would be one for 2015 though as this year is full.