Inspired by another thread where tiny theft is essentially de minimus and not a big deal... how about the opposite... if you found one million pounds in the street that belonged to someone else, would you keep it?
You can assume the following:
There is a 0 per cent chance you would get caught
You would have no problem using and spending the money (just forget about all the money laundering prevention laws here)
The money belongs to someone else
You have no idea who it belongs to or what they would use it for
So it is purely a question of morality not practicality. WWYD?