Every property already has a UPRN (unique property reference number). If you go on a council website and find a recent planning application it will be linked with this UPRN in the council's database. If I ever want to find a postcode I go to the find a planning application map and look it up there. I've not checked this in England, but it's definitely the case in Scotland. e.g. here's a random example; the entry for St Mungo's Cathedral in Glasgow:

https://publicaccess.glasgow.gov.uk/online-applications/case...