I’m sure an FOI request would reveal the lease costs with Titan, who provide the aircraft and (I believe) the crew and maintenance.
The requirement for provision of air travel at all is something I suspect many YACFers are never going to agree with, whatever justification is put forward. There is a tendency here to argue that if it can’t be done by Zoom or by bicycle, then it shouldn’t happen. But the world doesn’t work like that, and incremental changes in equipment and behaviour are what you’re going to get, whether you like it or not.
The A321s (I believe there are, or will be, 2 of them) have effectively replaced the 4 or 5 BAe 146s of 32 (The Royal) Sqn at RAF Northolt. Those aircraft were around 35 years old, were very dirty by modern standards, had a very limited range, but cost more than the 321s to maintain and fly. They also belonged to the RAF, so had potentially overriding military or Royal tasks that could render them unavailable to the Government. The A321s are brand new, have no military role, and are among the most efficient aircraft in the air.
I don’t have the data, but - Covid times aside - I suspect that diplomatic/trade missions are smaller and less common than they once were, despite the fact of Brexit meaning that they are rather more necessary than they were when we were in the EU and those things were an EU responsibility. In my days in the RAF, one or more of 10 Sqn’s VC10s would be pretty much permanently on Government duties. And yes, they had a posh fit for pollies/Royals even then.
The passenger VC10s disappeared in the late 90s, and by the time I joined Virgin in 1998 the U.K. Government had gone to a pretty much all-commercial policy except for a very few high-profile trips - PM to the United Nations, that sort of thing. To provide the flexibility and reliability of communication they wanted, 100 seats a day in each direction were contracted with Virgin to Washington. 100! At almost full fare; no big commercial discount because governments never negotiate anything in advance. About 10% of those were in business class, 90% economy. The daily cost in 2002 was around £100,000. Every day. That’s just one destination, though Washington was undoubtedly the biggest. That contract ended in around 2012, and they went to individual booking. I think that ended up costing them more! I don’t know what the situation is now, but I have no doubt that using the A321s is saving them a fortune.
