Sorry can't help with brands or suggestions as I'm on the wrong side of the pond.... but:
Be wary of buying any bike (used or new) that don't fit the touring bill. For example, changing gearing isn't necessarily a trivial thing (e.g. if you are fully loaded and touring, gearing that comes on a traditional road bike will be too high -and that means potentially changing all the drive chain which can suck out as much money as the bike). Also it's not just purchasing new parts -don't forget if you are not mechanically inclined you will have to pay someone to work on the bike.
If you are working on a tight budget and buying used, then it's definitely best to get a bike that is a match or the best closest match to what you want. Sometimes "bargains" can work out to be expensive.
One potential alternative is to buy a used '(rigid) early to mid 1990's mountain bike (mtb bikes of that era can have good road geometry) that is nicely below your budget. If you tour fully loaded (with good hills on the route!) then the mountain bike gearing is ideal -though you will spin out at around 20-22mph (which if you are touring, unless you are going down a hill, you're going to be lucky to achieve or keep up for long stretches anyway). Further, older mountain bikes usually have all the hardware -e.g. rack mounts. With the money you save, buy slicks, inner tubes and racks and panniers, or trekking bars or bar ends to extend your handlebar positions, etc. While I know this doesn't help you, it's not unusual for me to see touring potential used mountain bikes for under 100 quid here. This might be of interest -or inspiration -to you:
http://www.bikeforum...version+touring
Here's my conversion (since powder coated in black):
http://3.bp.blogspot.com/_G91oAUDO7iQ/SPag6Qil9DI/AAAAAAAAAGM/agmRZ5G39ZY/s1600-h/img46.jpg
Another alternative is to get a used hybrid bike, but the problem here can be the higher gearing.
If you think you might do touring seriously, I'd definitely advise to get the best bike you can and see if you can stretch your budget. If you think this is going to be more of a one off or that you might tour only very occasionally, then the used touring bike or converted mountain bike might be the way to go.
Good luck, hope you find a really good bargain!