Here is a list of some useful metal detecting websites.
The Crown Estate allows metal detecting on foreshores and estuarys so long as terms and conditions are met.
The research section of AddLand lets you see the boundaries of land ownership and in some cases the name (such as the farm name). Free functionality is limited but useful.
Home of the Portable Antiquities Scheme, containing a database of reported finds and their locations shown on maps.
A map of rural settlements and finds, filterable down by category such a coins.