I know that Wicked's US grosses are steadily in first place but where does the London production fall.

No one really knows. Whereas all Broadway theatres publish their grosses officially every week (except for Young Frankenstein), there is no such central, official process in London. Shows only publish their grosses when they can spin them into a press release. So while we may know exactly how much Wicked makes, thanks to the producers trying to hype the show some more (cynical, moi?), we really don't know where that falls compared to other shows that don't publish grosses.

That said, Wicked has said several times that it has broken records and is the best-selling West End show - but we must treat this as PR hyperbole. There is simply no reliable way to compare everything in the West End, even for the producers. Bear in mind, though, that Wicked has fairly high prices and one of London's biggest houses, so chances are that yes, it is making a lot of money.

Mark Shenton wrote a pretty interesting article about this a couple of years ago, and I'm fairly certain he's revisited it since, but can't find it anywhere.