Lost and found property is not the police's responsibility, however some forces do provide a service with regards to lost and found property. You can find out if your local force offer this by telephoning their non-emergency 101 number.
If they do provide this service, you will be given a form that entitles you to collect the property if it is not claimed within a specified period (except mobile phones and any other unsuitable objects). This period provides the owner with a reasonable amount of time to come forward and for attempts to be made to locate the owner, allowing for circumstances where a person may not have noticed that something is lost, or where they are on holiday.
28 days is a common period of time for this purpose, but can vary from force to force. Your local force will advise you of the relevant time periods when you report an item as found.