A valid point Graham, and I agree that it's not right for those on foreign licenses/plates to deliberately flout the law knowing that it will be too much trouble for the authorities to follow up (and don't even get me started on the "diplomatic immunity" fiasco).
I don't think the "on-the-spot" fines will be quite what people imagine them to be though, where a cop pulls over a speeding driver and money is handed over then and there. Such an arrangement would be unconstitutional under English law, since it would amount to a fine being imposed before conviction.
There are already cases in those areas where parking enforcement has been handed over to local councils and the latter are trying to tell people that the fine must be paid with no option to go to court.