Is it OK in the UK for mains sockets to be mounted upside-down?
The answer is that there is no specific regulation within BS7671 which is the British Standard for wiring regulations that specifies the orientation of the sockets. The reg that governs socket positions is 553.01.06. It states 


  A socket-outlet on a wall or similar structure shall be mounted at a
  height above the floor or any working surface to minimize the risk of
  mechanical damage to the socket outlet or to an associated plug and its
  flexible cord which might be caused during insertion, use or
  withdrawal of the plug.


It could be argued that fitting a socket upside down will cause the cord to double back on itself. However, Since it is mechanically clamped inside the plug its hardly a valid reason. The one item that could make it non compliant is dependent on its rated testing. If the electrical/fire testing for certification prior to been sold of the device for operation was never done in that orientation the thermal/electrical/fault ratings might not be relevant. There maybe a case to be made on its IP rating due to its reversed position but again its a very thin argument. These are all speculative ideas as pointed out the regs do not forbid it.

The reality is that it is acceptable but not good practise.