I've seen some rather amazing singers in London, but few of the shows seemed polished. Generally, the heavy-hitters such as the Lloyd-Webber or Mackintosh shows will feature the best talent, but then, even a small show like All You Need is Love or Song of Singapore will have fantastic singer like Issy Van Randwyck. The long-running shows are the ones that usually fall by the wayside. The biggest difference I have noticed would be in the dancers. The British chorus dancers usually are not as sharp and clean as Americans.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian