For some reason there seems to be a corrupted entry in some ISP's DNS Server databases relating to Britmodeller. Because of the way these databases are propogated across the net from the primary servers, it may or may not spread to your ISP's DNS server, and may possibly come and go if you are affected.
If it affects you, you can change your computer or your router's primary DNS entry to 18.104.22.168, which is Google's primary DNS server. This will get you into Britmodeller until the corrupted entry is expunged by the next round of updates (usually around 24 hours)
You may not be affected, but if you are at some point, at least now you know what to do :o)