If you really want answers to your questions, well MUFC has answered why English players have gone to Australia and not played 1st class cricket.
Regarding Callum Thorp, Durham would have needed cheap cover because they lost so many bowlers, non England contracted, to the national side and squad. They therefore couldn't afford to splash out on a high quality international bowler, so they took a punt on someone cheap who was willing to come and play.
Australians are as I'm sure you're aware always mighty keen to come to England. They earn good money in comparison to back home, and they get to spend the majority of their time buying drinks off their own countrymen so they don't feel homesick.
English players on the other hands often have jobs during the winter season, they go through university and play for their Uni sides and it is not always feasible to go abroad to play cricket if they have families or work related commitments.