Consistency of the mains frequency throughout the UK
National Grid (NGET) "Guidance Notes - Synchronous Generating Units" Issue 12 says


  Target Frequency
  
  If a generating unit is required to have a Frequency control device as specified in
  CC6.3.7 it must also have the facility to accept and act upon Target Frequency
  instruction from National Grid. In order to adjust electric clock time the Grid Operator
  may instruct settings of 49.95Hz or 50.05Hz. However, under exceptional
  circumstances, the instructed settings could be outside this range. A minimum setting
  range from 49.90Hz to 50.10Hz is required.


I suspect Andy's answer is correct concerning local fluctuations within parts of the UK connected to the national grid.

See Wikipedia on Synchronisation


  An AC generator cannot deliver power to an electrical grid unless it is running at the same frequency as the network. If two segments of a grid are disconnected, they cannot exchange AC power again until they are brought back into exact synchronization.




NGET Frequency Management includes several interesting graphs that may be relevant.




So, if a calamity occurs, you may briefly find that one end of the country is 0.05 Hz adrift from the other end for a fraction of a second. End users might need expensive and well calibrated meters to accurately detect that.