Although this is not a tracked service, Royal Mail do scan all packages when they collect them from us and they are scanned again at the point of delivery. This means that we can check whether a package has been delivered, but not before.
Royal Mail’s policy is that if the postman cannot hand the package to you then they should leave a card explaining that is was not delivered, it is then returned to the sorting office where it is scanned again to show a non-delivery. It is then your responsibility to contact the sorting office to either arrange a re-delivery or to collect it yourself.
Royal Mail recommend that you wait 15 days before lodging a lost parcel claim and that you check with your neighbors in case one of them agreed to hold the parcel for you.