The EU has banned all Angolan carriers from its airspace with the exception of a couple of aircraft from the national carrier TAAG which presumably get some special inspections and management while visiting mainstream countries.
Cameron's wonderful advisers have contracted this 747 from a secondary Angolan carrier, Sonangol, who to circumvent the ban on their country have got a US charter carrier to provide a registration for this aircraft. Isn't it great that our PM is supporting some grey market dodge.
I presume that, with the normal level of knowledge about aviation in the Prime Minister's office, they have no clue about nationality of airlines, EU bans, how to spell British Airways, or anything else. And our wonderful know-nothing Secretary of State for Transport can't advise them. They probably asked initially for the 747 to depart from London City.
I wonder how an Angolan company ever got offered such a charter in the first place. Of course, they are from a country where there is only one sure-fire way of doing business .........