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For that budget, you could look into buying a seperate, dedicated audio recorder, I hear the Zoom H 4 N is excellent, and you could combine it with a Canon EOS 550D with kitlens (although, you’ll need some accessories as well). You’ll have to change your workflow accordingly, i.e. you’ll have to add the sound in editing. But that’s how I’d do it.
I also know the Sony NEX cameras have mic accessories, but I don’t know how good they are, and they use the hotshoe.
Perhaps you should consider joining a tech and/or video site/forum, you can get help from real experts there. I’m on tweakers.net, a Dutch site, but I don’t know a UK equivalent unfortunately.