I would estimate £6,000; assuming that you pass in little over forty-five hours, train in something cheap (ie Cessna 152), pass all your exams first-time and improvise with some creative ingenuity for some of the other accessories.
For example, cut corners on expensive training aids by borrowing from the local library! You can probably do away with purchasing a flash knee-board by using a clipboard from the local stationary shop as well.