Older extension cords yes, newer ones have a larger gap between the earth hole and the edge.
I found this when testing an older extension cord which DH brought from his parents' house. Every other extension cord in my house had been bought brand new by me since 2009, when I moved in. His was god knows how old, because they keep stuff for years (and why not, if it works).
The ikea statement about "earthed and non earthed" plugs just stems from a misunderstanding of UK electrics. Pretty much all other countries have both earthed and non earthed plugs - the US ones which look like smiley faces are earthed, the ones which are just two lines are unearthed. Most EU plugs are either round with two round pins, and metal contacts on the edge or middle (earthed) or a long, squashed hexagon shape with just two round holes like a shaver plug - unearthed. Ikea socket covers are based on EU plugs which is why they are badly shaped for UK plugs - in fact a lot of socket covers sold here, you can see the outline of a US plug (slightly squashed hexagon, or rectangle with convex curved sides) or they are round like EU plugs.
The way the UK deals with earthed and non earthed appliances is to add a plastic pin in place of the earth pin. If you somehow jam open the shutters, you can plug an EU two-pin plug into a UK socket.
To the poster who asked about the round pin plugs, I found this on the website:
My house has round pin sockets, are these dangerous? Should I use covers with them?
Before the introduction of the BS 1363 13 amp socket British homes were wired using a variety of round pin sockets. If your wiring system is very old then you should seek advice from a reputable electrical contractor about testing and possible re-wiring. Some round pin sockets are still used in modern installations for specialist purposes, for example 5 amp sockets are sometimes used in lighting circuits to allow floor lamps etc to be controlled by a central switch. Round pin sockets used in the UK are now required to meet BS 546, which does call for shutters. However there are still some older versions without shutters in use, and these should be replaced. We have not been able to find any suppliers offering socket covers to fit British round pin sockets.
And the person who said they had no shutters - see the reference picture on fatally flawed's front page. The ones which are the exception are "MK" type sockets. These have different kinds of shutters but they are still shuttered. The modern type look like this: highfields-arc.co.uk/constructors/info/files/mksktext.jpg
Actually I've just realised that the ones in my old house were metal and didn't look like this at all, and one was in DS' room!
In the holes there were two metal bars or plates which I suppose get forced outwards when the plug is inserted. Hard to explain but if you have it you'll probably recognise it. Wish I'd got a picture now - would be interesting to email to FF and see what they say about them. They might have been old plugs, the house was Victorian.