Unfortunately, I doubt that you'll find anyone to employ you until you're 21.
To get full bus licence, without any restrictions, you must be 21. At 18 you're restricted to driving public service vehicles, with no more than 16 seats, on routes of no more than 50km. Although a few local bus operators use large minibuses for their services, it's still unlikely that they'd be able to employ you simply because their insurance will specify a minimum age for drivers. This is likely to be at least 21; some companies will, for insurance reasons, only employ drivers who are at least 25.
http://www.direct.gov.uk/en/Motoring/DriverLic ensing/WhatCanYouDriveAndYourObligations/DG_40 22547
Chris