If you came on a boat you'd be looking at a hefty train fare to get to London. If you flew to Stansted with Ryanair, it would probably cost around £15 to get to your hostel from the airport. Have you considered the Coach? Take a look at Eurolines website (I don't know the address, sorry) which would give you info and prices. It would drop you right in the centre of town as well. (Victoria Station)

I live in London so feel free to PM me with any other questions.