A pan is required if you think you might land with less than reserve fuel.
A mayday is required when you WILL land with less than final reserves.
Reasons why you would make these determinations?
Having made one diversion, and it's gone below minima, you may have sufficient fuel to go to the second. But you don't want to hang around. You don't want any delays, such as number 5 to land behind 4 dash8.
If you want ATC to help you, first they need to know the picture. A simple pan gets the message across very easily. It doesn't imply that everything is looking bad. It does imply that the crew are very much aware and in control of what is going on.
I wasn't flying yesterday, so I don't know what the weather was. But if I try to put myself in the same position mentally, and using a lot of supposition.
Forecasts may have indicated some fairly grotty weather.
The plog will be calculated as always with the normal fuel, and a choice of 4 alternates (for us, normally with the nearest alternate selected automatically, but the final decision rests with us).
Maybe knowing the sort of traffic you might expect at certain times of day and factoring in weather and local knowledge with that too, I would likely take fuel well above this computer calculated figure.
Similar situations can be encountered occasionally for London/Southampton/Bournemouth.
I've seen it happen a few times where the vis has dropped at Gatwick, Heathrow, etc. A quick listen to all the london weather built a picture of similar happening at Southend, Stansted, Luton, Southampton. It means a lot of other people may be in the same situation, so delays would be likely.
Perkin.
There is more to it than just where the company bases are. Newcastle may seem like a more appropriate choice than Teeside, but as we don't know the full details, we are hardly in a position to say if that was a good or bad choice. The crew operating had that information.