In reply to:

W for a shilling = double you for a shilling (betting)

That's what I always thought too - up until the moment I posted it on this Board, when I realised (a) I've never heard anyone actually say 'double you for a [insert monetary unit]' - the betting phrase is 'double or nothing'; and (b) I have heard plenty of mendicants say 'trouble you for a [insert monetary unit]'.