I was waiting at the wrong gate with a friend and by the time we realized this, they had basically finished boarding our flight. The attendant at the gate we were standing at called ahead and we sprinted across the airport but when we got there, they had already sealed the doors.
We spoke with customer service and they put us on the next available flight, but charged a flight change fee of $275 USD per ticket. That price is more expensive than a new ticket, or a train from London to Paris but we didn’t seem to have a choice because they would cancel our return trip if we didn’t update the ticket.
We have travel insurance through ISIC, but as far as I can tell it doesn’t cover stupid mistakes.
edit: airline was British Airways