happened to me a couple of times. On my way to LHR and a fatl accident on the roads mean i was 1hr late - as was everyone coming by road. BA ticket on Finnair, they checked that there were "exceptional circumstances" and booked me on the next flight to HEL via ARL. I was panicked, but they were calm and no problem. If you have a problem en route, call BA and they can probably do something to help. If you get through security and pass out in the lounge..... well, good luck!!