Dont go near BSM and AA and the likes, they charge far too much. All driving instructors are qualified to the same standards. Whether its BSM makes no difference at all to the quality of the instructor-- this depends on the person him/herself entirely. In fact, going with an indepent company, you may find better service as they need word-of-mouth to continue to succeed in their business, rather than a big name that sells itself.
I went with a local chap who was excellent. In general, I would avoid the big names they charge more and dont offer anything extra for it.
As far as your first lesson is concerned, your instructor will only take you down quiet roads and maybe a couple of main roads with close supervision (remember they have dual controls!). Most, if not all of the roads, will be 30mph. Most of the time will be spent on starting the car, finding biting point, moving off, gear change, steering etc
I didnt even know what a clutch was before i started my lessons, or which peddle did what so don't worry, this is NORMAL! Everyone is nervous on their first lesson, you're instructor won't make you do things you're not capable of. If you're unsure, ask and repeat.