Like many small businesses you are going to need a lot of help and support and a good place to start is the Driving Instructors Association. This association was founded in 1978 to represent the interests of the professional driving instructor. It is regularly consulted by all the relevant Government bodies, has representation in Parliament, and enjoys influence in Europe and throughout the world. Take a look at their website you will find lots of helpful information.

You will obviously be searching the net for ideas and how to get started, will you be part of a franchise or go it alone?
Here's a statement from one of the UK's leading franchises:

In today's fast moving society it is essential to be able to drive. The number of lessons taken by each learner to pass the driving test is increasing year on year. One study showed that those who passed the test had an average of 67 hours of driving experience compared with 49 hours twenty years ago. Therefore demand for driving lessons and professionally qualified instructors is growing rapidly with many instructors having waiting lists. (On average learners in Britain take more than 50 hours of lessons and spend more than £1500 to get their licence).
Source - Learning to Drive Consultation Paper (Dept of Transport 2008)

I don't profess to know whether it is best to be part of a large group or to go it alone, but what I do know something about is marketing and this will play a key role in the success of your business. Image is very important to any business so you should take some time considering your business name and how it will appear, on your car, in your advertisements and on your printed documentation.

One other important piece of advice is you will need a good accountant who can advise you on the pitfalls of starting your own business, make sure that you inform HMRC that you are commencing in business and understand what your taxation obligations are, then once you are earning be careful to set aside money each week/month to pay those obligations.

A good place to start for any business is your local business link they can advise you and point you in the right direction, at this stage all the professional help you can get will only help you understand which direction is best for you to pursue.

Once you have decided upon your name one of the next things you will need are some business cards, if you want to portray a quality image then ensure you buy the best available quality, check out this site they have lots of pre designed business cards and stationery all available for you to personalise and adapt to suit your needs. The quality is the best available on the net and their prices are very low.

I hope this information is of some help and welcome your comments and experiences in becoming a driving instructor.