I did London-to-Brighton and London-to-Southend on a stock saddle on my Specialized Hardrock Sport and by the end I was in what I can politely call "some discomfort". I'm not quite sure what a "Brooks-shaped arse" is but I bought a B17 Special and compared to the previous saddle, this is almost like the difference between wiping your arse on silk rather than sandpaper. I often do a 20-mile trip from work to home and I have no soreness at all when I get home whereas I'd be slapping the Sudocrem on with the previous one.
At the time of purchase and when I fitted it, I did question why I spent £80 on a saddle -- even berating myself a little. There are lots of people gushing over how good they are and I'd have to say I don't regret spending the money at all. Given how much nicer it is, it'd be the first thing I buy if I get a new bike.