In short, using plug socket covers is more dangerous than not having any.
British power sockets are fully regulated by government regulations. The BS 1363 standard for 13 Amp sockets is required by “The Plugs and Sockets etc. (Safety) Regulations 1994”. Sockets must be safe and include internal shutters to prevent children from poking objects into them.
Unfortunately the government does not regulate socket covers; this means that perfectly safe sockets are made unsafe by inadequately designed and totally unnecessary socket covers.
The requirement for sockets to be child safe and include a shutter mechanism dates back to a remarkable and far-sighted wartime committee which resulted in the publication of ‘Post War Building Study No. 11 – Electrical Installations’ in January 1944. This study looked at post-war electrical needs including a new type of socket and stated: “To ensure the safety of young children it is of considerable importance that the contacts of the socket-outlet should be protected by shutters or other like means, or by the inherent design of the socket outlet.” The BS 1363 standard was introduced in 1947.
British sockets are designed so that the holes for the plug pins are too small for even a new-born baby to put a finger in! (It is also worth noting that you would have to reach 1cm deep into the socket before contacting a live part.) The purpose of the internal shutters is to prevent children poking objects into the socket. The shutters are automatically held in place by springs and are only opened when a plug is inserted into the socket. Most sockets have shutters which are operated by the insertion of the earth pin (the largest pin at the top of the plug). Some sockets require the insertion of more than one pin at the same time. The basic method of earth pin operation is perfectly safe providing that children are not given access to tools which are the same shape as an earth pin and which could be used to defeat the shutters. Unfortunately a socket cover is such a tool! Socket covers are flimsy and bend, and they invariably have pins which are shorter than a real plug. It is possible for a child to experiment with inserting a cover upside down, opening the safety shutters, this exposes the child to the danger of severe electric shock! It is common for parents to report that their small children (sometimes as young as 5 months old) can remove socket covers without difficulty.
Once a socket cover is in the hands of a child they can easily insert it into the socket in a way which defeats the internal shutters and makes the socket very unsafe.