I've tried doing some basic google searches and I've come up with very little. There's no "Created by" or "Written by" credit in IMDB either. Who wrote this? Who came up with the original idea? Who, at the very least, came up with the outline? There's no way the show was just improvised the entire time.
I should be doing something else besides posting here. You too... Just a reminder.
All three of them, Coogan, Brydon and director Michael Winterbottom. Wikipedia has more info if you look up the "The Trip (2010)" on Wikipedia, but the premise came about after Winterbottom directed both of them in the movie based on "the unfilmable play" A Cock and Bull Story where they got round the unfilmable part by playing semi-fictional versions of themselves mulling about behind the scenes.
A few years later the three of them while discussing new projects noted that the success of A Cock and Bull Story was down to the scenes where they were messing about. They took it from there...
There's a second series out this year called The Italian Trip. It's more of the same but a tour of Italian restaurants.
Not seen the movie for about a decade but if I remember right there is a hilarious scene that goes on for about ten minutes where Steve, Rob, Gillian Anderson and a bunch of extras bitch about what food was available from the movie's caterers.