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For touring, comfort on long rides, and folks who are not obsessive about being the fastest (or looking like they are) you will want a bike with a relatively long head tube and a shorter top tube.
Considering your size, I wouldn't think you would want or need a "touring" frame (like a Surly---(and I know the Surly aficionados will scream))----short tubes are inherently stronger so a bike which might be too light in my size might be really strong in your size.
I think you probably want a modified "sportive frame. "Sportive" means whatever someone thinks it means, but generally it is used to refer to longer rides at slower speeds than races ... Grand Fondos, big charity rides, "B" group rides (as opposed to "A" group rides where everyone is Type A personality, and needs to be the first to finish.)
Most bikes are good for a lot of things anyway ... but if you plan to do mostly around-town pleasure rides with some occasional touring, a "sportive" frame should be perfect. You could still load it down with four panniers, a bar bag, a trunk, and three bottles .... but since you aren't planning to cross Africa or something, no need to get a tank.
(I have done longish, fully loaded tours on "sports" frames with absolutely zero issues .... just avoid "racing" frames because they are so short, you will be kicking the panniers and fighting to keep the bike going straight all the time. Sportive sounds perfect.)
Now I have to go look at the Mercian site ... Look what you guys started.
EDIT: Based on whatever my tired old brain could decipher, Mercian calls the type of riding you want to do "Audax," as opposed to "full touring." You would have to ask them, but I'd imagine "touring" would have longer chainstays (more stable with big loads, more heel clearance, but harder to turn) and a slacker head angle (again stable when loaded but slower-turning.)
Probably someone there can work with you to take a whole lot of your money, and probably deliver a bike you really like---but only the money part is guaranteed.
