Quoting shaolinmonk from 11:40, 4th May 2007
Salvage laws are not that simple, for someone to legally 'salvage' something on the beach they must first register their find with Customs within a certain time period (i forget exactly how long) and they can keep it unless the owner then makes a claim. That is why the salvagers in England got into trouble, had they registered their find and the owner hadn't tried to recover their lost items all would have been fine.
Therefore I suggest you go and report your loss to the police who will be able to let you know if anyone has claimed salvage rights.
Salvage has to be declared with the Receiver of Wreck who is part of the Maritime and Coastguard Agency and if it remains unclaimed after a year it becomes the property of the Crown. They have to dispose of it, usually by auction although in most cases they don't bother if it isn't of value to them and the finder is allowed to keep it. In the case of several BMW motorbikes washed up from the MSC Napoli, those which were reported were reclaimed and most of those which weren't reported were seized by police.
With regards to a barbeque left on the beach I doubt the person who removed it is going to consider whether it was above or below the tide mark and then spend the time filling in a form to declare their find.