The TV updated its channel guide from the local multiplex system. Your local TV transmitter obviously covers parts of both East Sussex and London, so both local news broadcasts are carried, but the default in your area is to receive the broadcast for East Sussex. The TV possibly just latched onto the strongest signal being received locally, or perhaps East Sussex came up first because it would be ahead of London in an alphabetical index. At least you have the option to select which channel you want to view.
Similar systems seem to operate when using BBC iPlayer software on a computer. It tries to determine your location from the internet address and may refuse to play programmes from other regional areas, or if you are using a virtual private network (VPN), that allows you to pretend to be anywhere in the world, as far as the computer and internet are concerned.