Hi
I'm Chairman at Cambridge - I'll PM you my phone number, give me a call and ask any questions you like.
But to answer a few of the points:
At Cambridge we have bookable training - so you can book training by the half day with a ratio of two students per instructor. That will give you a good amount of flying for your time on the airfield (but you will be expected to help get the aircraft out and around the launch point for the rest of the half-day).
For someone like you, who wants to learn to fly, I wouldn't recommend a trial flight - they are for people who more typically want a 'gliding experience'. Instead, come along and book some training - a half or full day, or even a one week course. We do that for a membership fee of £25 per day, plus flying at club rates.
In the long run gliding is both cheaper and (IMHO) more challenging than power flying (and I speak as someone who still has more power hours than gliding, just). At the early stages though, it's cost benefit isn't as marked - since as has been said, you do a lot of launches per flying hour.
Paul