up up and away

Was in London working today so had an early start and was not back until after 10pm. Last time I was in London I missed my homeward flight due to a security scare at the time of the G20 meetings. So this time I booked the later one at 8.30pm just in case there were any hold ups. Of course this time I was there at 5.00pm and could easily have got the 6.10pm one. Can anyone tell me why you have to pay almost £150 to be transfered on to an earlier flight when you have clearly already paid for a ticket and there were empty seats on the earlier flight??? So.....spent three and a half hours sitting working on my laptop in London City Airport. Still, it had been a good day. By the way, it was really wet in London!