Hi, welcome to OpenStreetMap.

I'm afraid that you can't use the Royal Mail website's postcode finder as a source for OSM, as this breaches their copyright and is explicitly prohibited by their terms and conditions - https://www.royalmail.com/postcode-finder-term-conditions-en

You can use postcode centroids for some addresses, which are published by Ordnance Survey as OSM-compatible open data in their Code-Point Open product. These are available as an overlay in the iD editor which you are using.

There is more information on UK addressing and postcodes here - https://osm.mathmos.net/addresses/