In reply to gammarus:
Well, if you're in the Northwest, you aught to consider the Hewitt steel bikes (
http://www.hewittcycles.co.uk/) as well. You will have a limitless choice (series production, semi custom and made to measure if you want, in race / audax / light touring / full touring / expedition configurations) plus a bike from him will come with a proper fitting session, and that will do more for comfort than any particular frame material or joining method. The aftersales will also be second to none.
After that I would also recommend thinking about traditional wheels. A three cross spoked wheel is more compliant than radially, blade spoked 'modern' wheels which translates to more comfort and durability. With something like OpenPro rims and a reasonable set of hubs, there's no weight penalty either.
Whatever frame you get, make sure it's capable of easily taking 25c tyres and ideally 28c (for lightweight touring) with proper mudguards. Even when fitness riding, a good quality 25c tyre again does wonders for soaking up lumps, bumps and pot holes. Also make sure you've got enough braze-on fittings to fit any combination of racks you think you might need.