(Speaking as a Police Officer here!) I don't honestly believe that any police officer would steal money handed into the station - they have to give you a receipt for handing it in, and then if they pocketed the cash and then you rang up to enquire if it had been claimed or not, then it would be discovered that the money had 'disappeared', and the officer would lose their job. Its just not worth it for a police officer to ever steal something from custody. ALWAYS hand it it, and it will be returned to you if not claimed.
For those who said it's not theft because you found it, unfortunately it IS theft. It doesn't belong to YOU, and therefore it's theft. "A person commits theft if he dishonestly appropriates property belonging to another with the intention to permanently deprive the other of it" Section 12, Police and Criminal Evidence Act 1984.
For those asking where the cut off point is, well, it's not theft if you make 'reasonable steps to locate the owner', so if you found £5 on a deserted road at midnight and kept it, it wouldn't be theft because you couldn't really make any steps to locate the owner, but if it was £5 in a school playground at pick-up time, for example, and you pocketed it, it WOULD be theft because you would have had an opportunity to locate the owner and not taken it.