You're still thinking about that Stoke-on-Trent lady who picked up a £20 note dropped on a shop floor by another customer and kept it. She was prosecuted for theft and pleaded guilty. She was given a conditional discharge and ordered to pay £175 costs so she is running at a considerable loss.
Generally, where do you legally stand when confronted with the prospect of a quick profit? Unless you are passing a shipwreck and have the ability to pocket the hull or are in game with your three year old daughter, forget the idea of finders keepers because you too could earn yourself a criminal charge. BUT you will not be guilty of theft unless you acted dishonestly. So if you keep the money or other property you have come across and you genuinely thought you were legally entitled to do so then that won't be dishonest and won't be guilty of theft. In the case of property other than money which you find, it would not be dishonest to keep it believing it had been abandoned by the owner. The crazier the idea that you genuinely thought you could do what you did, the more likely the court will conclude you have been dishonest. Hiding what you have picked up under your raincoat or looking around to check that nobody is watching would suggest dishonesty. And you may have an uphill task explaining why you did not hand over what you found to a member of staff or at least told them what you had done and given them your name and address. And why not report the find to the police?
If convicted of the theft of money the court is likely to order you to repay it to the owner. Should it not do so then the owner can sue you for its return in the county court.