I've dug out some contemperaneous cuttings and so on to refresh the ageing memory.
It's much as said above by various people; it was going southbound and was told to land almost as soon as it entered Libyan airspace.
It could have just turned round at cruise speed without descending and headed North again, perhaps landing at Rome until it was sorted out, we all thought.
The two Governments and BOAC kept shtum, but the "not-in-the-know-but-not entirely-stupid" conclusion in the Press and elsewhere was that the landing was pre-arranged. Certainly there was talk of one or two spooks on board by coincidence. The two who were taken off were not popular in UK either; in fact I seem to recall they were being deported although nothing I have confirms that.
I'm wondering if the Captain was among those who went to Bahrain a few years later to fly for GF?