For what it's worth, I really like the London cast CD. I was lucky enough to the show both Off-Broadway and in the West End, and on stage I thought that they the talent of the casts were about equal. Unfortunatly, the London CD suffers from some appalling production and sound issues which affect the chorus particularly, but it was perfect on stage. As I'm British, I don't tend to notice any slip in accents
I also prefer 'Mine all mine' to 'Inside my heart', and like the addition of dialogue which makes the CD different enough from the OOBC to make it worth listening to. However, I agree with Lome about the weak 'Hey Freak' (which is just a variation on Mine all mine, and not sung that well), but I still play both recordings quite often
And no, they didn't use a specifically British accent for Bat Boy in the UK, though the line about learning the language from the BBC was still there and got a laugh, as did Devon May's delivery of "It's the social event of the season!"