I would try circusair too as at your age any experience you get will pay dividends in the end. Then you can go and work for a real company
At 15 you will be looking at basic office duties and not ramp duties but don't be deterred by this. It's a first rung on a sometimes long ladder.
Also keep plugging away if you get negative response and get a list of other companies or airlines at Gatwick. As some have said, make a phone call as this will normally get you an answer straight away.