Then either he's misunderstood what you're asking, or he's talking bollocks.

As mentioned in the post above, it's the registration document (in the UK the V5c, also known as the logbook) which is important. As mentioned above it doesn't have to be in your name, but I would recommend it as foreign officials will start asking extra questions if it isn't. The guy you're buying the car off will insist on transfering the registration to you, because if he didn't then you could drive through speed cameras, get parking tickets etc, and they would be sent to him.

You need some mates living in a shared house in the UK who don't give a shit what mail comes through their door. This should include any Aussies over on the obligatory 1 or 2 year working holiday visa. You give the guy selling the car this address and your name. The new registration document (v5c) gets posted to this address and has your name on it along with this address. For all purposes concerning the vehicle, you now live there. The fact that you don't live there is irrelevant. There is no such thing as officially residing at an address. We don't have this concept in the UK. You live where you say you do. Because you're immediately leaving the country, there is nothing going to be sent to you at that address of any importance apart from the v5c itself. You only need tax, MOT and UK insurance long enough to get out the UK.

I take it you're doing one of those banger rallies, probably to Mongolia? Seeing as how loads of Aussies do them every year with UK vehicles without any problems, I think that you're looking for problems that don't exist.

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UK to Mongolia 2009, on a DR350