This information is taken from the Portable Antiquities Scheme's leaflet Advice for Finders of Archaeological Objects, including Treasure.
It is an offence to metal detect without the written permission of the Secretary of State for Culture, Media and Sport on a Scheduled Ancient Monument or on land within zones designated as Areas of Archaeological Importance (i.e. those in Hereford, York, Chester, Exeter and Canterbury). Some local authorities also have byelaws prohibiting the use of metal detectors on their land. It is against the law of trespass to metal detect without the landowner's permission. If you see anyone using a metal detector on such a site, please remind them of the law, as their actions reflect badly on the hobby of metal detecting.
The Environmental Stewardship Scheme is operated in England by the Department for Environment, Food and Rural Affairs (Defra). It is a scheme in which farmers and land managers enter into ten-year land management agreements to achieve specific environmental objectives. In Environmental Stewardship agreements, two conditions specifically relate to the use of metal detectors.
Remember: It is against the law of trespass to metal detect on private land without the owner's permission. Any finds that you may make on private land without permission belong to the landowner, and you may be prosecuted for trespass.
You must have the permission of the owner or authority responsible for the land before you begin metal detecting.