Explanation I heard for the ridiculous innertube change prices was that the cycle shops are not interested in changing people's tubes- they'll happilly do it for the extortionate fee, but not for a "reasonable" one. Their mechanics' time is better spent elsewhere...
Near Liverpool Street Station to just south of Tooting Bec common. About 7.5 miles in 26 minutes... I may have jumped a few red lights (but reasonably!).
If the miche sprocket includes the carrier, I'll have the 18t one. :)