The BBC Question Time format works quite well - last time they weren't able to persuade the party leaders to sit down together before the same audiance, but sticking them on different night facing an equivalent audience, asking questions about anything they liked, (no prior notice), worked quite well.

Clicking on this gives you the latest programme, if you've got RealPlayer or something like that. No top politicians most weeks, but it gives the flavour. (You get to it from this page)

As for those so-called debates you've had in the States, in which the candidates stand up alongside each other and take turns to perform a highly coreographed dance - that just appears a total waste of time, and rather an insult to your voters.