sounds like in your case you had already checked in, and it was security that caused the miss. they can see in this case that it is not totally your fault, and might be more sympathetic.

i missed a flight from CDG one day, non-flex tkt. (i should have been there earlier but i prefer to blame the train). missed the cutoff by about 2 mins, after running the 2km or so (what it felt like) from the station to the BA desks. is it just me or is CDG a real sh-t of an airport? zero sympathy from any BA people, despite the fact that the inbound was late and hadn't even arrived yet, that made me very angry.

anyway the on topic part - i was told that normally they would try and put me on the next flight if there was space, but that this was not a set policy. i was on the last flight of the day so tough luck. this was however done in a very rude unhelpful and dare i say, very french manner. i remember thinking at the time that if i really wanted a hot french woman to verbally abuse me, i would have been on AF to start with! who were cheaper anyway...

to conclude this story then, i tried to rebook for the next day, but the BA staffer on the ticket desk told me it would be cheaper to go on the eurostar (i was poor at the time). so back into paris where i crashed on a mate's hotel room floor, he laughed at me a lot, then got a eurostar ticket the next day. you guessed it, it was way over the cost of a BA ticket, it was about £220 for a walkup economy seat.

that was not a good weekend... i haven't missed a flight since then.