I have several positions on British politics (and several more on US politics) at Intrade.com.
The site lets you post your own odds in an exchange style fashion on financial markets, political, legal and other current events. 2 which have gone my way recently are (in the US) Giuliani for Republican presidential nominee and a short position on Gordon Brown for next PM who I think was way over priced at 80%, I expect he will win but thought 80% to be overdone given the whole Milburn scenario.
I prefer this type of election trading to the indirect version through the markets as you can lay prices (no spread) and be certain of the correlation between price and event.