Wasn't Oxford - Cambridge tried a few years ago by Rangemile Aviation? Would some form of Air Southwest style Plymouth/Newquay setup not work using B1900s or J31s, for example Oxford-Cambridge-Edinburgh or Oxford-Cambridge-Newcastle etc?
I trained in Oxford at the very beginning of my training for my ATPLs etc and the airport seems to have a good catchment area and a lot of potential. Someone like Manx2 or Eastern would be a good starting point, maybe with Flybe coming along later?