Y'all do things funny over there in the UK. Why would someone blacklist an entire ISP, not just one particular IP address?
Out of idle curiosity, how many ISP's do you have, anyway? Where I'm at in the US you're lucky if you get two choices (either the cable company or the phone company). And many places don't even get that many choices, lol. 
i have noticed it when ocassionally using a mobile connection in gernamy or using certain mobile providers when i am underway in poland. hell, i have even had the same issue with an italian isp, and that from a day one, when i had a residency close to rome few years ago. therefore i suspect that a whole isp may be blacklisted.
that may not be understandable for an american, who never needs to travel abroad
