Right, you can get to London Bridge by tube without any down escalators (or any escalators at all). You can travel by District/Circle line from Victoria to Canon Street (one flight of stairs down and up, seven stops, twelve minutes) and then take a National Rail train from there to London Bridge (one stop, three minutes). Or you can stay on the tube to Monument (one more stop), alight there (stairs only) and take a stroll across London Bridge to the south side.
OR... you could get off instead at Embankment (three stops) walk up to Charing Cross (five minutes) and take a National Rail train from there to London Bridge (two stops, five minutes). Easy Peasy !!!