Recall a pretty windy day, very low cloud and and even lower U2, COCISS and appearing in my wing mirror just above the trees and buildings as I drove into wind along the A420 outside Swindon. He drew and stayed level with me for quite a bit - didn't have all that much overtake
. Im crap at judging height and I know it's a deceptivly big a/c but I'd be surprised if he was any higher than 3-400 ft... clearly though, he was on his way to/from the US and had must have had altimatry problems as they never operate out of UK...