I would turn it in to the local Police Station. I can show that easily.
1. In England/Wales it is a crime to steal 'by finding'. Disregarding my own sense of self-value, even if I was a thief my reputation is worth more than a lousy £600!
2. Despite some police being naughty, most are good, and if this is not claimed then I can make claim to it after a few months.
3. If the owner claims it, then I might, just might, get a reward.
4. I don't need it. We have got more money than we need already. This is because we live in such a tiny home, with very low overhead costs, and enjoy a very simple life. So why nick money that we don't need?
5. If a person doesn't hand it in, they will know...... They will always know...... that they were wrong.
This only counts for me, in my circumstances, where I live.
Forty years ago my answer would have been different.