IIRC unclassified shooters can use 28gm, but can't win prize money. You'd be classified on the day on two random rounds ex four and be given a class accordingly. Theoretically Ed you could be classified in A and have the highest score beating regular A class shooters using 24gm and that wouldn't be right.
Rugby is only UT, ABT and Helice. Nearest OT would be Nuthampstead near Royston, NCSC at Bisley or Beverley CTC in East Yorkshire. I'd say Nutty for you just off the A10.
Buy a slab of 24gm ya tight wad.
If you're going to Nutty let me know and I'll come over.
Edit: just checked the Nutty website, there's no registered shoots during November or December.