Gliding is great fun and teaches good handling and airmanship skills which I think transfer usefully to powered flying. I've only had a few trips in gliders though.
The thing I found frustrating is that the gliding clubs I have experience of expect you to spend a whole day helping out in order to get flying. This is a great social experience and very enjoyable but if you only have half a day spare it's hard to get airborne; picking a flying club and booking a slot that suits you makes life easier for the terminally busy.
If you're likely to be near London for a while, Clacton do a Super Cub PPL and that's a good way to learn proper stick and rudder skills. And more fun than a Cessna.
If there's a discount for paying in advance, take it but use a credit card so if the school does go bust you will be able to claim against the card company.