Ah, the people's republic of New Ash Green. I know it well. So long as you are in England is all that matters. I am doubtful whether there is a sufficient act of abandonment for you to assert ownership by finding. If, however, he was to sue you in person for conversion you would have a counterclaim (and I think quite possibly a set-off) for the money he did not return.

More exciting is whether it would be theft. I think, technically, there is a risk that it might be but I would expect S 2(1)(c) of the Theft Act 1968 (look it up) as amended to afford you a defence.