Definitely try gliding. It's not only one of the cheapest ways to get into the air, it's really good fun, and it will teach you a great deal about flying which will very likely make you a better pilot if you later want to fly power.
If you enjoy it and stick with it you could be flying solo in a couple of years, and for pocket money costs (well, maybe Saturday job costs...)
Your mum is definitely wrong to say it's nothing to do with flying!
Remember the airliner which made a forced landing in the Hudson River? The pilot is a glider pilot (he obtained a glider pilot's licence at the age of 14!) and it's my guess that without his experience in gliders the outcome might have been a whole lot worse.
By the way, in this country, there is no gliding licence. You will be allowed to fly solo when your instructor thinks you are ready (not before you're 16 though).