A 2009 Medway Council FOI reply says the public are not allowed to use metal detectors on council land. It does not mention the foreshore specifically and is old.
Covers
Medway estuary shore (Gillingham Strand, Riverside Country Park, Hoo Peninsula)
Watch out for
Crown Estate or private foreshore is not council land; check ownership.
What the source says
"Medway Council do not allow the public to use metal detectors on their own land."
Tap any spot in England on the UK Detector map to check it against scheduled monuments, SSSIs, nature reserves, National Trust land and access land in one go. Free: 3 checks a day.