Picking the best driving instructor or driving school for your driving lessons does not necessarily mean that you need to go with the most expensive option, there are plenty of high quality instructors who offer an affordable service. However, it is your responsibility to make sure that a low price is not indicative of a poor service. Often going with the cheapest option available can offer tempting cost savings but if the low cost is coupled with a poor level of tuition you may find yourself paying out more in the long run through the cost of repeated tests and additional lessons.
The first thing you need to be sure of if you decide to go for a low cost driving instructor is that they are sufficiently licensed and adequately insured. Under UK law anybody taking money from you in exchange for driving tuition has to approved by the Driving Standards Agency (DSA). They will either be an Approved Driving Instructor (ADI) and display a green octagonal badge in their car or a licensed instructor under training and display a pink triangular badge. Obviously the risks of paying someone who is not approved by the Driving Standards Agency are that the level of tuition they give you is not regulated or checked, they may in fact end up teaching you bad habits that will end in you failing your test. Always us a DSA approved driving instructor.
When deciding on the right instructor for you try to speak to a couple to get a feeling of how well matched you will be. Ask them for information on how long they have been operating, how busy they are, how flexible they are with lesson times, how well they know the area around the test centre you will end up using etc. Try to gauge how well matched your personalities are, the last thing you want is to take driving lessons from someone who you cannot get along with, it will make you dread your lessons. A good driving instructor should make you feel relaxed behind the wheel and make the process of learning to drive an enjoyable one, rather than a high pressure environment.
A decent way of finding a good driving instructor in your area is to talk to you friends or family members to see if they can make any recommendations. A recommendation from a trusted person is worth far more than any review on a 3rd party website so you should always go down this route first.