No, that makes sense. From a security point of view, you just block everything inbound, and only open what is needed as it becomes needed.
But that's only for traffic that originates outside the firewall/router. For traffic that originates inside the firewall going to the outside, I'd normally expect pretty much everything to be open, with the firewall allowing reply/acknowledgment traffic to pass through as well. I'm sure there's a technical term for that, but I'm buggered if I can think fo it right now.
If it's not being explicitly forwarded to your PC, then it's almost definitely blocked by default at your router. Otherwise it would be an open route into your network.
I doubt BT universally block anything inside their network before it gets to your router.
I guess I'm in an odd position that my internal IP at work is actually my external IP and as far as I know there none or very few restrictions on ports.
I was talking more about the ISP managing the router, but yeah, I'm forever being prevented from FTPing and such. And it's a fucking miracle if my Lync client works.