I shoot regularly here in the UK with Eley Hawks 21g First cartridge and find it a superb cartridge in my Reilly hammergun. The Reilly is not a particularly light 12 bore, but I just prefer the lighter load and lessened recoil when shooting clays, especially rounds of 50+. If I can't shoot with these, I use a RC 24g load of 7.5's - certainly not heavier than this.
DGM - the lead shot debate is certainly a concern, and although the main shooting organisation here, the BASC, has come in for criticism for what is seen as a concillatory stance on the issue, I would far rather have them at the debating table than outside it, unable to influence the discussions. It behoves all shooting folk here in the UK to communicate their own point of view on this as well to support BASC - with a more immediate concern and focus being arguably the Govt firearms review later this month which comes hot on the tradgedy of Cumbria.