Probably because he lives in London & Stapleford is much, much closer & more accessible than the nearest gliding clubs.
Sigh..
I did mention holiday course, did I not? He is fourteen years old. Instead of faffing around doing umpteen trial lessons and learning not much at fairly great expense, he could actually put the money his parents are providing to good use and fly solo much sooner, and gain good stick and rudder skills.
Just because Stapleford is close does not make it invariably the best option. Save up the cash, go for a week at a time, and a achieve a result. Then fly power when he is old enough to get a licence.