Roger - there is NO such thing as a 'fuel emergency' in the UK. I would have thought you would know that?
There is an emergency caused by lack of fuel - Pan or Mayday, depending on how short you are, but arriving at xxx with insufficient to hold is down to the Captain of the a/c to handle.
This a/c was probably not in an 'emergency' fuel state at the time it went away, so declaring an 'emergency' would not be a good idea! It may be that this a/c loaded 'sufficient' fuel at departure and used it en-route for whatever reason, and it is by no means uncommon to arrive with less than a 'comfort' figure in tanks and 'hang on' and see how the stack thins out before making a diversion decision. JAROPS allows you to have NO diversion fuel at all in reasonable weather at LHR.
Allowing those who load insufficient fuel to 'jump the queue' by asking for an expeditious approach would open a huge can of worms.