Well, my ISP is Be as well and I've actually contacted them about this. They gave me some commands to enter using the telnet client in the command prompt, but they haven't made any difference.
It's claimed that I've had strict NAT all day, and I have spent absolutely ages trying to sort this out, which is why I'm convinced it's either something at Be's end causing this, or buggy NAT detection in the game... very annoying.
It's absolutely undeniable than the ports I've listed are open, and everything else is as I say it is. I don't mind having to do it, but I expect it to work when I do.
Does anyone have a link to anywhere that officially listed ALL the ports required by the game and steam? as errors in that list are about the last explanation other than blaming my ISP or the game itself.