Failure to perform yet asks money for it!

In Jul 2015 I booked tickets for me and my husband to LA via London 4 months in advance of our travel date (Nov 2015). A couple of days after booking I received an email which states that I can pre-book seats at an additional expense. Since I knew I was pregnant, and because we are both quite tall, I decided to take up this option. The total cost of pre-booking the seats for our 4 international flights was R5600. The seats I selected were the two single seats behind premium economy/business class, giving us extra legroom.However the actual seats we got when we flew were in a row of 3 and I had to sit in in the middle seat for all the flights and climb over people to get out.When we arrived in LA I sent an email complaint to BA, to receive an answer only yesterday. The answer politely states that the T&C's state that configuration can differ from plane to plane and that they can't guarantee the seat you actually end up getting. But why then charge me R5600 for it??? I performed my part by being diligent and paying a huge additional amount. BA did not perform and essentially stole R5600 from me - and now hide behind the small print. I am a loyal customer and I am extremely disappoint