There was a Government Minister in the news this morning I believe it was David Lammy. He reiterated that U.K. Nationals were informed in April this year to leave Afghanistan, and that’s when the evacuation started, five months to get out first on scheduled flights, when these were unable to continue it was charter flights and ending with military flights of the last week.
There was indeed a last minute rush, which no doubt was fuelled by the Taliban taking over the Country and ousting the legitimate Government. Not sure why US/U.K. intelligence could not have predicted this event?
The U.K. could not have stayed at Kabul Airport any longer as it was controlled and protected by US Forces. Once they were going home (leaving approximately $85 million of equipment including aircraft, black hawk helicopters and a multitude of armoured vehicles and weaponry, so the Taliban will either be extremely well equipped or rich on the proceeds of selling it to the highest bidder) the U.K. had no choice other than to withdraw.