A local driving school. Check around and ask friends. The cheapest may not necessarily be the best choice. Be aware that, if you buy them a low insurance group car such as a Citroen C1, they can get insurance much more cheaply while they are learning and are supervised, than the premium will be once they are driving unsupervised and pose a much greater risk. Quinn Direct is unusually cheap for learners but premiums can treble as soon as the test is passed.