So the verification procedure insists on government ID, but doesn't work with one of the two forms of government ID generally available in the UK.
Most UK citizens will currently have no access to a Government-backed 'digital identity service' (and won't until the Gov.UK Wallet comes out).
There are third party digital identity services such as the Post Office EasyID scheme that are backed by government documents, but Apple doesn't work with them. (3/7)