Re: like a passport or driving licence.
Why would someone trust scans of their passport or driving licence to some random third party.
In the UK, only place that gets a scan of my passport is my bank - and they do that on paper, not electronically, and its shredded when finished with.
But with some random "verifier" a lot harder to be sure they will not keep the scans of what are very useful identity theft data (genuine age (not fake DOB given to most web sites), passport number (useful on travel site exploits), genuine name & address etc. )
And lots of UK people lack driving licence and passport
.. and as chivo243 implies, very easy to use someone elses documents, e.g. smut searching kid uses image of a parents ID to create their age verified user account