British socket outlets are amongst the safest in the world if they are in good repair.
Electrical sockets that are manufactured to BS1363 standards (including multi way adapters and flexible extensions) must prevent the insertion of foreign objects into the socket tubes by an interlocking shutter mechanism. The BS1363 standard also sets a distance which a plug pin must be inserted before coming into contact with live parts.
With a socket constructed to this standard and in good repair, even the smallest child cannot come into contact with a live part, with their finger or other foreign object.
The problem with socket covers is that they are not constructed to comply with any electrical safety standards. The pins on the socket covers may be of a different size to those on a BS1363 plug and could damage the socket.
Also, if the pins on the socket cover are too large the cover could be difficult to remove, causing a pin to snap in the socket tubes, normally the long (earth) pin, causing the shutters to be jammed in an open position. What was a safe socket has now been rendered dangerous, by inserting a non-compliant object into it.
In 2016 the Department of Health withdrew the use of socket covers from all NHS and Social Care premises and stopped supplying them for use in a home or residence.