I am a detectorist in the UK and the law is that if the item consist of more than 10% precious metal and over 300 years old, its declareable. Single coins of silver and gold are not unless it is classed as a hoard (3or more as a rule in close proximity or pot, etc). As for the Roman/Saxon/Celt/Viking finds as long as they do not meet the reequirements above, they are not declarable.
All items that could be classed as Treasure have to be reported to FLO (Finds Liason Officer) within 14 days. The item then goes through the treasure process and is either returned to the finder or purchased by a museum (purchase price is split between finder and farmer)
Hope that helps
Neil