If I thought it was genuine I would print the e-mail, take it the local sorting-office, with ID (as they do request) and ask for the parcel there.
However, Royal Mail always leaves a card if they cannot deliver, but I think do charge a small fee for re-delivery or re-direction. I certainly trust it far more than the courier firms, which are designed primarily to deliver to business not domestic addresses, and can be the sloppy ones.
So I'd be very suspicious of an e-mail supposedly from Royal Mail – especially as I have not given it my telephone numbers or e-post address!