For most of the regular fullbore long range matches in Great Britian, they do use ammo from Radway Green Arsenal. They ain't allowed to use handloads.
The arrangement under which Radway Green (RG) ammunition was supplied to the GB NRA came to an end in 2008 and since then ammunition for GB NRA competition (for Palma-style matches) has been supplied under a commercial contract with RWS. Their product uses the Sierra 2155, the rest of the components being their own. The result has been a step change in performance, but also in cost. Most of the club team match circuit has also switched from RG to commercial products, usually from smaller domestic producers, and handloads are increasingly being accepted in club open/individual competitions. The RWS contract ends soon, possibly after 2013, so we could see a change of supplier (or rules) after that time.