As has been said, it is you that keeps slipping in this word "maximum", which does not exist in BS1363.The maximum that they must permit clearly means that they are not required to accept any more than this. That's what "must" means.
"Must permit two conductors" means just what it says - it does NOT mean "Must not permit more than two conductors".
I agree that if all one knows about a socket is that it is BS1363-compliant, one has to assume that its terminal capacity may not be any greater than the (minimum) specified in that Standard. However, if the specification of a particular BS1363-compliant socket indicates (as MK ones do) that it is 'rated' to exceed that (minimum) required terminal capacity, then it makes no sense to not believe that 'rating'?
Kind Regards, John