I don't live in the UK, I live in America. In May of 07 I took a business/personal trip to London and from there made a side trip to Derby especially to see the Mercian shop, which wasn't much of a shop as my brain had imagined, but the people that worked there were wonderful. So I had one custom made to mine and their recommendations, and the bike was shipped to my home about 3 months later. So I don't know where in the UK you can get used bikes. Though another friend of mine went to London about the same time I did and he found a bike shop in London that sold new and old bikes and bought a 81 Holdsworth in near mint condition, but where or what the shops name is I couldn't tell you.
Of the bikes I mentioned earlier you're not going to go wrong with any of them. Personally I like the Jamis because it has an old school flair to it, the others look to modern, but my personal taste prefers the older more artistic old school looks.
As far as your specs goes, The frame material you choose is a great frame for touring, unfortunately lower end bikes do not go to that level of a frame, they either use Reynolds 520 which it the modern kin to the 531, or 4130 which is the Asian equal to the 520. The 531 was world renown for rugged reliable touring frames so the 520 won't be any worse-neither would the 4130. The fork, since 531 is no longer in production you would have to settle for whatever steel fork the factory puts on it, and that would be fine!
The most important derailleur is the rear, where I would like to see nothing but Deore, but when you buy a production bike it comes with what it comes with, sometimes an LBS will swap you components and give you credit for what came on the bike to what they put on but that will bring the price up. The very lowest lever I would settle on the rear is Tiagra. Then the front whatever because it doesn't do the exact shifting nor as much work as the rear has to do.
Headset? I don't sweat the head set stuff, their very reliable and next to the hubs the least likely component to fail. I've never had a headset fail not even on cheap Walmart kids bikes. I like Ritchy Logic, Cane Creek, Chris King (though I think those are way over priced), FSA! And others.
Chain; Shimano, SRAM, Wippermann, KMC, and Rigida
Bottom bracket; Shimano is fine, as is Chris King, SRAM, TruVative. some of the FSA's, RaceFace.
Your front hub is a great choice, you may or may not find those on a touring bike in your price range.
Rims again are a large open arena, but you need to look for double walled box shape rim with dual eyelets and at least 36 hole rim 40 is best, anything else and you compromise strength needed to carry touring loads.
Tires you choose are decent, but there are a few others like Schwalbe Plus. Tube? Anything goes
Saddle; I haven't heard of the saddle you mentioned, but the two most popular touring saddles are the Brooks B17 and the Terry Fly.
A lot of the stuff you mentioned is unique to the UK area and not found here, but I'm going to give you some web sites that will help you with your education on touring:
bicycletouring101.comTableOfContents.htmOpen ↗
kenkifer.comtouringOpen ↗
cyclocamping.com87 1 cat.aspxOpen ↗
struck.usBicycle Touring Tips.htmlOpen ↗
myra-simon.comtips.htmlOpen ↗
There's a lot of reading, but these were done by people who have years of experience touring, so please take the time to read and learn as much as you can. Obviously do your own web searching too.
