I'd say 'on' or 'about'.
'Chap' is fairly common, less so among the young. Some use it only for men, I use it for both. However, the idea that everyone walks around saying 'I say, old chap' is just for the cinema. It's used in an ironic way nowadays.
Traditionally the 's' was the BE form, but we use both- often in the same text. I'd say it's hard to say which is more common.
You can use both, though in the imperative form, it is more common not to use 'up'. We also use 'bell me' as an imperative quite a lot now.