The main issue on the ground was people not having the correct paperwork that was being demanded.
This was dealt with by staff in London and soliders in Kabul who weren't used to handling such things - the embassy staff who would shipped out immediately leaving them to deal with it.
There are reportedly thousands of emails which werent even looked at.
This then becomes about how much time staff had to devote to answering media inquiries about dogs, having to sort out emails for help versus anger emails from animal welfare activists as much as where the dogs went. You then have staff at gates dealing with this convoy coming through - thats extra vehicles and extra times opening gates and holding up others also queuing in the process and checking all the relevant paperwork for the dogs. And then loading onto aircraft. Even if this was voluntary and on breaks it still would have taken precious time from other tasks people may have done on breaks in crisis management stages and it certainly led to resentment amongst some service personnel (both british and american). Remember it was Americans on the Abbey Gate that some had wanted to shut permanently shortly before the terror attacks occurred.
You couldn't just roll up to the entrance and get in. There were a whole load of processes and hoops to jump through which involved a large number of people to facilitate.
Saying it was just about dogs taking up space is very naive and doesn't reflect logistics at all.
Its a HUGE operation. At the time they had thousands of people who had been processed and were actually already in the airport who had to wait for hours for their flights. It was apparently up to 48 hours. Even the sky news guy who by his own admission was fast tracked had 7hrs waiting for his flight.
I doubt that people could have been put on Pens plane because the army had to have it guaranteed it would land. Until it actually did there werent sure. Then they would have wanted a civilian aircraft off the airfield asap. I note that they were sending off heat tracers from military aircraft as they took off because of the threat of heat seeking missiles to take out military planes which were thought a geninue risk.
They couldn't take the risk of adding people to another aircraft because passenger manifests for who was on board generally have to be done before departure even in those situations, so they know who they have actually taken (so they know they haven't left people behind in the airport). If Pens plane hadn't turned up they would have had to rejig other passenger manifests to accommodate them elsewhere.
These are the type of logistical and operational issues that had to be considered in a very short space of time.
Most people won't consider them because 'awww poor dogs didn't take up much space and went on their own plane' doesn't require much to comprend and to contextualise in the wider operation to evacuate.