As others have pointed out STN is a swine to get to by public transport unless you live in London or the Cambridge area.

Coming from the north, I've found it easier to get to Gatwick than Stansted by public transport - doubly so now that the Thameslink station to Gatwick is at St Pancras

If you find the hire companies that use STN you should be able to get a one way hire as a not too extortionate cost

I'd also check into flying via Amsterdam to wherever you are going