I don't think its necessarily true that an extended delay to an aircraft that's already declared minimum fuel will result in a Pan call. Say the delay is 30 mins and min fuel is called - But now that delay then extends by another 20min after a temp runway closure half way through the 30min delay. So thats still 15mins flying time + fuel for an approach thats still available in the tanks (and for completeness, final reserve fuel too).
So say the aircraft is in the LAM hold for LHR, a div to STN is possible without touching final reserve fuel. So no need for the Pan call. Similar for other locations in the UK...
To make the point though - This is purely a hypothetical situation and some extended delays may well need to declare an emergency! But often there may be other options...
Totally agree with DVM's final paragraph.
Ed to add...
When I use the alternate fuel for holding then I commit to a single approach. I'm not committing at that stage (the hold!) to the destination, just to a single approach...