Briefly, all UK laws have to be followed. You are breaking the laws of trespass if, in England and Wales, you search on land without permission. On parkland you run the risk of prosecution for wilfull damage if you dig huge holes. Follow the Code of Conduct and use a small trowel and replace divots neatly.
The Treasure Bill came into force on 24th September 1997 and you are obliged to follow it. Write to the Department of National Heritage 2 - 4 Cockspur Street, London SW1y 5DH and ask them to send you a copy of The Treasure Act 1996 Code of Practice (England & Wales), this book is free of charge and will tell you all you need to know.
The other law of importance to metal detector users is that concerning scheduled ancient monuments. It is an offence to use a detector on a protected scheduled ancient monument unless permission has been obtained from the Secretary of State for the Environment. When seeking permission from the landowner ensure that you ask if there are any protected sites on his land.