I've ridden Bristol Bike Fest on a single speed and don't think I would have been any quicker with gears.
Wiggle 6 at Catton Park had a few more hills and I probably lost a few overall places riding single speed.
I didn't even attempt Set2Rise single speed, it's too hilly for me, so I used the Rohloff.
So, what's Margam Park like, I've never been there ?
There's no separate class for single speed, so, unless it's very fast and flat like Ashton Court, which seems unlikely in Wales, I'm guessing there's no advantage.

