Would avoid any routes ex-ESH that also operate ex-LGW. It's a short train ride to LGW and as mentioned above, LGW carriers probably do it cheaper and quicker.
To a lesser degree, the same applies with OXF and LHR (although, of course, there are no no-frills carriers at LHR and no direct train link).
Maybe speak to Brighton City Airways about ESH-POX if you can. It folded after 2 months.
Good luck with your studies Yvonne, and let us all know your progress.