@PeterH: They do it here in the U.S., and it is very common now. My ISP (Cox Communications) blocks port 80 requests to non-commercial ip addresses. I'll bet in the U.K. there are ISPs that do the same. I've dealt with several users from different countries that have port 80 requests from the internet blocked. They had to resort to port 8080 or the like.
Those dynamic DNS services are the reason the blocks are being implemented. It normally requires a commercial account to host a website, and the ISPs want their money for doing that. Besides, even if your ISP does not block port 80, I'll almost bet it is in violation of your contract with your ISP to host a public website with a residential account. It is here.