Now hold fire just one minute!
Riverboat, before anybody jumps the proverbial cannon, let me clarify things for a sec.
I am not suggesting for one minute that somebody who simply declares a 'Medical Emergency' is going to ignored. That was never my intention and is NEVER the case. Summarising my point- the ONLY recognised declarations in the UK are MAYDAY and PAN. If a pilot suddenly develops a problem with the aircraft then we aren't going to get caught up in pointless discussions as to exactly how impossible their task is going to be. On the other hand, if the a/c is fine, they're 10 minutes from the holding point and need to queue jump for a sick pax and delay another 20 aircraft then if you simply declare a medical emergency then in 95% of cases you will be nudged in the direction of, or asked if you're declaring, a PAN.
In these days when the London TMA is as busy as it is, it is often pushed to the very limits and as ATCOs we are 100% focussed on getting to guys and gals from one side to the 'tother without touching. With delays the norm, if an a/c is in any kind of emergency and needs special treatment you can typically multiply our workload by a factor of 2 or 3. Now for genuine situations we are only too willing to do so but the implications for ourselves and the other 20 aircraft in the vacinity have to be considered and accounted for.
It is not purely the ATCOs decision either. If every a/c that has a slight medical problem received preferential treatment there would be carnage every day. If there are 20 minute delays and somebody needs to jump the queue then fine, but the only way we can be sure is by clarifying the situation. Lets not forget the requirement to obtain authorisation from the 'Powers that Be' above mere ATCOs as well.
Basically, if you declare a Med Emergency the ATCO will think 'Is he/she declaring a PAN', and time will be wasted. If you decalre a PAN with a Sick Pax, then we can all concentrate on getting you back on the black-top.
It is very much like the old fuel emergency situation. I'd wager a fair few pennies that the ATCO's response will be - 'Are you declaring a PAN/MAYDAY?'