Re: Nobody cares about planning evacuations
and no military wants to be seen planning to lose a war
Any military that is competently led *should* be planning for likely scenarios - including emergency withdrawal.
Yes - the utter collapse of the Afgan Government was unexpected but it shouldn't have been - what did they think would happen when the US/UK publicly shoved them under the bus? It's a foregone conclusion that the majority of the Afgan army would either desert immediately (if they even existed given the corruption of the higher echelons - there were entire units that were being paid that didn't actually exist) or run for safety as quickly as possible.
There were (of course) units that did stand and fight knowing that they could at least take some of the Taliban with them rather than waiting to be tortured and killed if they surendered.
 Same thing happened with the British Army in the 1800's - which lead to the establishment of a whole corps of officers and administrators to check such things..
The abrupt US withdrawal *was* communicated to the UK DOD well before it happened so their planners and strategists should have been 'what-if' planning well beforehand. But that would involve the DoD actually being competent and not just full of time-serving wasters and superannuated senior officers.