Thanks all. I did wonder about runway length but if they can squeeze a 747 in and out of Cambridge (~6500 feet) I reckoned they could get a lightly loaded A330 in and out of Northolt.
FWIW and
AFAIK previous PMs in the post VC-10 era have used chartered BA jets - notably Concorde on one G7 meeting. So of course the French had to use one of theirs too. Or maybe (or perhaps more likely, probably), it was the other way round. In any case it was a great piece of one-up-manship: you may have a dedicated VIP transport, but *mine* is a *lot* faster.
But we are currently alone amongst the large industrial nations in sending our big-wigs abroad in a military jet rather than one painted appropriately. Many would commend this frugality; personally I think that it sends quite the wrong message out to other countries, many of which have a culture where appearances are taken much more seriously than in ours.