Originally Posted by sipples

Same here in Singapore. BA's Web site is blocking all inbound requests from this IP address or range of IP addresses, it appears.

I've already ruled out other potential causes: IPv6, something client-specific, etc. For example, coming in via a VPN works with nothing else changed. Southwest.com has the same problem albeit with an "Access Denied" return message instead. If BA.com and Southwest.com share the same hosting provider (or some aspect of such), that's a clue.

I've reported the problem to the ISP, but what can they do about it? Their pipe is obviously working (see above re: VPN). BA.com is returning a Web page, after all, just not a very welcoming one. And the whole Web is working. I also called that +44 number and tried to file a problem report or to speak with somebody who knows how Web pages work (and don't), but, naturally, nobody answering that phone number has any idea how to handle such calls. If you're going to display an error message and ask somebody to call it'd be nice if the people answering the phone know what to do.

Update: Yes, BA.com doesn't like at least on incoming IP address. I was able to change that IP address through a non-VPN method, and BA.com likes the new address. Southwest.com still doesn't, so these two Web sites are blocking different ranges. I kept a record of the blocked IP address if anybody from BA wants to contact me offline.