Found the AIC
http://www.ais.org.uk/aes/pubs/aip/pdf/aic/4P058.PDF
you may need to register for free to view.
The salient points are.
8 Pilots should be aware that although every effort will be made to expidite their arrival, a call such as "Fuel Emergency" has no status in the UK and ATC cannot give priority to an aircraft with a shortage of fuel unless an emergency is declared
9 A radio call prefixed by MAYDAY for distress or PAN for urgency will ensure priority habdling but the aeroplanes actuial fuel state should reflect the seriousness of the emergency call. A commander should only make such a call when he belives the aircraft to be in danger, not because the fuel state has fallen below the ammount needed to comply with formula given above.
Well I was trying to copy and paste but in their infinite wisdom the flipping AIC (a published public doc) is password protected. Muppetts. Apologies for spelling errors in my re-typing.