The important point is to only metal detect where you have the landowner’s permission, anything taken away without landowners permission is theft. Regardless of permission it is illegal to detect on protected sites, such as those defined as Scheduled Monuments, Sites of Special Scientific Interest or military crash sites, and those involving human remains. Detecting without permission (“Night-hawking”) causes the metal detecting hobby serious reputational damage, and could be used as justification to restrict metal detector ownership.
If you are new to metal detecting and do not yet have any permissions, then the best places to go metal detecting are the foreshores that are owned by the Crown.
The Crown estate has given permission to metal detect on their foreshore as detailed in this document. (https://www.thecrownestate.co.uk/media/2762/terms-and-conditions-of-metal-detecting.pdf) The following link to their website contains more detail as well as a link to their Foreshore and Estuary Map showing permissible locations.
Be careful though to check local bylaws as some local councils have the right to manage the beaches and may have their own regulations on metal detecting. In theory these will be good sites as people will have lost coins and rings, however they are popular detecting sites regularly picked clean by local detectorists. The salt water and sandy conditions do provide some unique detecting experiences and offer a great place to practise.
If you are interested in discovering artefacts, then rural sites near habitation and ancient roads are ideal. The best sites are near were there has been past human occupation, such as fields and woods near farms and villages, as there is a higher chance that things have been lost there. It is wise to do some research looking at field systems, old roads, tithe maps and lidar to build up a picture of what has happened in the past. This knowledge is useful when approaching a local landowner for a permission because it shows that you care and not just any Tom, Dick or Harry. (No offence meant to Tom and Dick in the club, at the time of writing we do not have a Harry)
The technique for detecting a field varies from person to person, some prefer to detect the edges first and then across the middle, others the high ground. The key is to try and understand the landscape, how it was used and evolved. There is likely to be more signals along paths and around entrances to the fields. The challenge is that these areas often have more iron waste like gate fittings, wire and bits-off-a-tractor (BOAT) potentially shielding good signals, and surfaces near gates are harder to dig. You are more likely to discover artefacts in arable land that has been recently ploughed as the ploughing churns up objects that may have been too deep for easy detection. Archaeologists are understandably nervous about metal detection and digging when finds are taken out of context, however ploughing has been proven to be highly destructive, so as long as you are properly reporting finds, and hold off and call for help when you find a hoard or something significant then you are rescuing our history.
A good place to metal detect is on Club digs. The club will have gained the necessary permissions and you can get a lot of useful advice and tips from other members who are happy to help.