I've got return tickets on Thameslink Great Northern from the Ely direction, via Cambridge, to Dartford. On the return leg, would it be possible and permissible to start my cross-London transfer at Tower Hill? It's listed on the National Rail website as one of the cross-London transfer stations, but I am guessing that this is because it's near Fenchurch Street. However, one could also travel from Dartford via Woolwich Arsenal, DLR to Tower Hill, then an out-of-station interchange to Tower Hill LU. Then to King's Cross via whatever route (e.g. circle line is simplest). So it would surely be acceptable to take that route (via Woolwich Arsenal and DLR, then Tower Hill), but what about just starting the return journey at Tower Hill? If that's not an option, then I'm assuming there should be no issue at all in starting the Cross-London transfer either at Cannon Street or Bank? If I don't need to pay the extra for LU journeys that are included in my ticket, then I'd rather not do so.