At present there are two metal detecting organisations that represent the hobby i.e. N.C.M.D AND F.I.D (National Council for Metal Detecting and Federation of Independant Detectorists)
Both have web sites which can be found at: http://www.ncmd.co.uk/ and http://fid.newbury.net/

In a way UK DETECTOR NET could be called an organisation because we are here to help metal detector users, both beginner and experienced.