MK Electric has been making electrical fittings since 1919 and shuttered sockets since 1926. The company has gained a reputation for high quality products and for many years their version of the 13A socket has incorporated a shutter mechanism that needs all three pins of a plug to be in place to open the shutter. They have done this because, like other quality manufacturers, they want their sockets to be as safe as possible. All good quality electrical fittings are rigorously made to carefully considered standards drawn up by groups of engineers with many years of experience.
The MK socket is designed to suit a standard 13A (BS1363) plug - not the other way round. Other responsible manufacturers design their sockets in exactly the same way. This means that any non-standard device which is pushed into the socket may not fit, and it could quite probably damage the socket permanently and dangerously.
This is not a defect in the socket design - the socket will perform correctly with a standard plug; it is simply not possible for the socket designer to anticipate what some irresponsible manufacture of a non-standard plug-like device might produce.
This is what all the blanking plug manufacturers whose products we have been able to find have done - they have ignored the carefully thought-out standard for plugs which has served well since 1947 and produced an article with the wrong size or shape of pins, (apparently, being wrong is good enough for them), in a material which is unspecified and untested. This shows either they simply don't understand the British Standard, or they don't care.
As an example of how even the best of sockets can be compromised by a non-standard device, look at how the MK shutter mechanism works. Note, clicking on a picture will open a larger version in a pop-up window.