A lot of people have offered helpful advice through this thread and I would like to reiterate some of it. I am a flying instructor but am not going to say where as I don't want to be accused of advertising
mr/ms conaty - Choose your school carefully and your instructor even more carefully.
I would be ashamed of anyone at our place who was grumpy and unhelpful to a customer who "just" came for a trial lesson. Any instructor should treat you as someone special - a trial lesson is often a real highspot for someone who may not be able to afford to do any more flying. Unless you say that you just want a jolly, you should get a proper pre-flight brief on the effects of the controls and then, when airborne, have a chance to see them demonstrated and try them all for yourself. When I get people who are dead keen but can't afford to go on, I encourage them to come up to the airfield at weekends and chat to our members, read our magazines etc. I have no interest in "snaring students" as I have more than enough work and we have plenty of students and hirers precisely because we treat people properly.
In an earlier thread, someone suggested using the trial lesson as a way of evaluating a school's and an instructor's attitude to potential students - and I know that some of our recently joined students have tried two or three schools in just that way. They found it helped them to decide.
Good luck wherever you decide to learn.
ps I'm glad I don't have to work at this mythical school where they can do the whole licence for £3K - presumably the instructors there are working for free ! We are comparatively cheap for the south-east but I always tell prospective students that they are likely to spend about £5000 and they shouldn't believe any school who tries to tell them differently.