I had the same problem with Amex yesterday and went through a similar series of checks to convince myself I wasn't missing anything obvious. Trying to buy 2 business class seats to Hong Kong on ba.com, kept getting rejected saying that the address was wrong, eventually got kicked out of the system and sent back to page 1 where I'd have had to put in all the passengers' details again. Faced with that, I gave up and bought them on cathaypacific.com which was very straightforward with an Amex card (although did require the entire UK postcode to be entered without spaces, but did tell me this was the problem). The relatives will just have to suffer a better airline than BA (which they initially preferred) but since I'm paying I figure I can pick the easiest booking option and I'm pleased not to be giving BA the money if they can't get their website to work.