American English has “Buttfuck, Idaho”, possibly substituting more polite names, and/or other equally sparsely populated states. If it’s abroad, it becomes Buttfuckistan or some equivalent.
Not sure about a British equivalent, but I’ve seen “Invernowhere” used in a Scottish context (there’s probably a swearier version this was bowdlerised from), and Dylan Thomas named an imaginary Welsh town Llareggub (read it backwards).