Another benefit of a slightly larger school is that, should your Instructor have their car off the road (or be unable to work because of illness), there's a chance one of the other Instructors may be able to cover your lesson, so you don't miss out on any driving. This can be especially useful if you have your test and your Instructor is unable to make it.
Choosing on which Driving Instructor will teach you to drive is a difficult one. The best way is to speak to friends and family first and see if there's anyone they would recommend. Using this method, you get a better idea of what the Instructor is like, before you actually start lessons. Failing that, browsing the search engines for driving schools would be a good plan. Get a list of several people, and phone them up. You will normally get a good feel of the driving school from the way they deal with you on the phone. If they sound dismissive then think about looking somewhere else. If, however, they sound interested in your needs, and ask you questions about previous experience or any requirements you may have, then they would be worth considering.
Remember, there's nothing wrong with changing your Instructor if you feel you don't get on with them. Everyone learns better when they enjoy the lessons!
For more information on BTI Driving, and the Instructors that work there, take a look at http://www.btidriving.co.uk