In many countries, socket outlets do not currently have built-in safety shutters to prevent access to live electricity with the plug removed, and in those countries using a “safety socket cover”, or “socket protector” may well be an excellent idea to help prevent children accessing live electricity, provided they are properly manufactured.
In the United Kingdom, however, British Standard socket outlets to BS 1363 have for decades incorporated safety shutters which are slowly opened as the plug is inserted.
The Fatally Flawed web-site discusses the issues that can be encountered using certain safety socket covers with BS 1363 socket outlets, which have apparently included instances of certain safety shutter systems being defeated if a cover is improperly used, or perhaps breaks.
It is certainly prudent for individuals and businesses to consider carefully the potential consequences of using socket covers with UK socket outlets, and balance these against the risks of not using them, which should take into account the safety measures already in place.