With my regulatory hat on ........
The UK 'law' on plugs and sockets is implemented through Statutory Instrument 1768, "The Plugs and Sockets etc. (Safety) Regulations"
These Regulations re–enact with modifications the provisions of the Plugs and Sockets etc. (Safety) Regulations 1987 (S.I. 1987/603) and make provision for the first time for certain requirements to be satisfied in relation to appliances.
www.legislation.gov.uk
This specifically requires socket outlets to comply with BS1363-2 plus amendments.
The standard defines not only construction requirements but also the plethora of tests that must pass for the design to be approved.
It requires products to be tested and approved (type testing) by an accredited test house, to the standard, before they are put on the market, and requires marking on the product to identify the test house to show compliance.
It is not permitted to CE mark plugs and sockets (now going to be UKCA since Brexit), 'self declaration' is not allowed for plugs and sockets, so the only legitimate route to market is through notified body testing.
The Socket under discussion has a spurious CE mark. I didn't see the mark of an accredited test house anywhere, and as Lucien commented on the 'instructions', I don't think any test house would have passed the instruction leaflet anyway.
I don't believe there is anything specific in BS1363-2 to say that multiple earth terminals must be connected (I could be wrong), so that doesn't help the cause, but it is plain that supplying these things is against regulations, so I believe Trading Standards could take action.