Gloucester to Thame? I think that's a longer drive than most will be prepared to do in my experience - that's hours rather than minutes.
My suggestion would be to start with a smaller run, more central to one area. The Kidlington > Burford runs we used to do worked well as you had a pub that did food either end and the driving time, through twisty b roads, was about 20/25 mins - the a40 for two hours isn't a terribly exciting prospect if I'm honest.
I'm not knocking your enthusiasm, I just think that you'll need to do a run near Gloucester and/or a run near Thame so as to not end up costing people hundreds in fuel/expenses