Sounds like a great trip!
Regarding the bike, I think the only advice I'd give is that there's no such thing as a standard feature list on a touring bike, and there's wide variation in the small details that can really make a difference when on tour. So, I'd suggest getting a relatively cheap, second-hand tourer (but a decent one still, so that you can make a fair assessment of things) as soon as possible and start doing some weekend miles on it. In the course of that process, you'll work out stuff like: how much kit you're going to be taking, how you like to load it and what you're going to load it in, lever placement/gearing/brakes, position/geometry/bars/saddle. When you're armed with this experience, you can then go and buy the bike that will take you on your trip! You might find that the bike you bought serves the purpose very well; on the other hand you might find you need something else (in which case you can buy off the peg or get a custom steel frame for reasonable money from the right builder and build it as you like).
I'd just add something else though. Building your own bike from the frame up gives you two big advantages. First, if you built it, there's a much better chance you'll know how to fix it! Second, self build gives you the option to use cheaper/different components. For example, 8-speed drivetrains make a lot of sense in this regard: cheap components; easy to find spare parts in far flung places (e.g. chains); more robust. The big bonus about touring is that it's possible to put together a good bike for very reasonable money, particularly if you make clever equipment choices and/or look at secondhand options.
Some websites you might want to look at:
I'm guessing you might have already found the crazy guy on a bike website? If not, take a look!
Roberts & Dave Yates make high-end custom frames. Not necessarily what you want (for price reasons, though if budget allows...!) but great if you can pick one up secondhand (my tourer is a secondhand roberts).
The CTC website has a good touring forum and is the best place to pick up secondhand touring bikes (ebay otherwise).
Spa Cycles in Harrogate is a great touring shop. Also build good wheels at good prices.
The Surly Long Haul Trucker is a very well designed touring frame at a great price. Actually, it's probably ideal for what you want given budget constraints (http://www.spacycles.co.uk/products.php ... 3s143p2258
) and you can probably build one up for quite a bit less than a new Super Galaxy.
Steve Goff, Bob Jackson, Paul Hewitt might be places to go to get relatively cheap custom or off-the-peg touring frames.