On the question of work experience, there is certainly no harm in asking, either by telephone, formal letter or doing so in person. Bear in mind first impressions count so make sure you prepare to make a positive impact (know before hand what you want to get from it, ideas as to what you want to do while you're there and look presentable... etc).
As for payment, pay after every lesson rather than pay in chunks of money (eg. don't pay £xxx for X number of lessons). Pay as you fly.
On the money side, it won't hurt to get a job (if you don't have one already). At your age you can fairly easily work for at least one day a week in order to save up. If you fly once a month, and yet save more money than you spend, things are looking good.