For ATVs, this means a full car or motorcycle license or a provisional driving licence with a provisional entitlement to drive a quad bike or tractor. Please note that you must also have completed the relevant theory and practical tests for the category of vehicle you wish to drive.
Driving a quad bike on a public road without the appropriate licence is a serious offence, and you could be fined or even disqualified from driving. For more detailed and most up-to-date information, it's best to consult the official government guidelines or visit the website of the Driver and Vehicle Standards Agency (DVSA) in the UK.