I would try this approach at the ticketing counter: "Hello, I'm in a tough situation. I need to get back to the UK now rather than Aug. 23 when my flight is but I don't have the 100 pound change fee. I'm desperate and I am happy to standby until you have a flight with a lot of empty seats. Maybe you could put me on one of those flights and maybe even then be able to sell my ticketed seat to someone else??? I have heard of young, inexperienced travelers missing their flight home and the gate agent taking pity and putting them on the next flight w/o a change fee. If they ask you what your tough situation is you can smile sheepishly and admit that you just didn't bring enough money with you to last you the trip.