That is a crazy amount of money to change flights, no doubt. I am a
travel agent but in the US not the UK. Here at least, the catch is this, you can change your flight for a fee (usually $150 but was $250 for the tickets you purchased according to pp) PLUS any difference in fare. So if I were to request a change to travel dates for a ticket I purchased for any reason at all I would have to pay the change fee plus the difference in fare which can definitely add up especially if the change is being made close to travel date and for a weather event when other are trying to rebook flights and therefore increasing fares.
I have been in your shoes with American Airlines myself when I was in Turks and Caicos and Hurricane Tomas came through closing the airport for 3 days. I called and tried to change flights before the hurricane and they had not waived the fees yet. If I proceeded with the change I would have had to pay thousands of dollars so I did not. 12 hours later the flight was cancelled and I was re-booked for free. Traveling during hurricane season is tricky. We had to stay longer and even though we got to change our flights for free we still had to pay the resort which was an unexpected expense. I always tell my clients to purchase insurance. If I had followed my own advice I would have been reimbursed the extra expenses caused by the hurricane. Stupid me!