WWE is huge in the UK. I suppose a large part of that is because it's so rarely there. They'd easily sell out Wembly.
That said, I think it's more likely we'll get a SummerSlam than a Mania elsewhere. I think SummerSlam 2012 could work from the UK, as it'd be the 20th anniversary (

) of the Bret/Davey SummerSlam.