If you wish to metal-detect on land which is not your own, you must get permission from the landowner before you do so. If you metal-detect on land without consent, you may be charged with a criminal offence and you could be sued for damages. Earlier cases where prosecution has taken place has also prove that you may might lose your detector as well.
How do I find some land to detect on?
There are basically three choices - Rallies, Join a club which has land available exclusively to members , or ask permission yourself at farms etc.
Good place to start and to try out your machine! Lots of modern houses have loads of building material and junk. If you have an older house you may make some interesting finds of historical interest.
Parks and Commons
A number of local authorities have specific policies restricting the use of metal-detectors on their land. You are advised to contact the County Council and relevant District Councils as whether you will be allowed to detect in your local park ect will depend on the attitude of your local Council towards metal detecting.
Older parks and commons have been used by people over the years for a number of reasons - fairs etc - so there must be a large amount of finds beneath the surface. Good places to search are around very old trees in parks and areas where people will have sat.
UK DETECTOR NET has an excellent article on beach detecting written by Phil D. This will provide you with all the information you should need regarding detecting on beaches. It is at the link below:
Ploughed Fields - only within the depth of ploughing
These are probably the most preferred sites for detectorists for making good finds covering every period of history and are normally easy to dig. The reason for this is that they are being regularly turned over by the farmer which brings new finds to the surface and top soil. Because of this though, many of these finds are damaged by the farm machinery and are also in danger from corrosion from agricultural chemicals.
Make sure you know where the farm boundaries are and check with the farmer if you can detect on drilled crops or not. Be aware of field drainage - some fields have buried pipes under the ploughsoil to run water off into ditches.
Tricky one this and the subject of some animated discussions between detectorists and archaeologists. For the detectorist the advantages are that you can search these sites almost all year round and finds will probably be in better condition due to lack of ploughing etc.
From the archaeological viewpoint - they would prefer detectorists to avoid undisturbed pasture or other grassland where there are likely to be significant archaeological deposits.
For example: if a field has been ploughed for five years, and then set to pasture, it would be seen as a responsible practice to detect on it since the artefacts are unstratified.
Footpaths and Woodland
You don't have a right to search public footpaths - they are rights of way only and you must obtain permission from the landowner.
Apart from lots of modern rubbish you are likely to find coins and items dropped and lost by walkers. The older footpaths (some are hundred's of years old) - those which link villages to local landmarks and churches are worth examining. Its best to search the area of ground to each side as well as the path itself.