There's no formal mechanism for registering the location of an IP address. IP addresses are assigned to network providers and they in turn will have a registered address. In the case of my 90.x.x.x address, this is allocated to Easynet, and their address is given in being in Brighton. The IP address of the exchange equipment is also assigned to Easynet, with the address being in London.

The tools which determine the geographical locations of IP addresses use different tricks of varying cleverness to work out where an IP address is actually located. In some cases they don't even get any closer than the right country. My suspicion is the case of Sky Broadband addresses, they use the central point of the area covered by the exchange. And they don't always seem to get that right either.