Because of the shortage of IP addresses many batches of them are being reallocated. They were originally allocated to one location, but have been taken over by another.
Plusnet appear to have acquired many from Venezuela and from parts of the US. They claim the addresses have had their new locations re-registered, and say that many businesses don't update their information. Hence ebay may accuse you of being in another country and display incorrect postage or airlines will direct you to their website based upon the original IP address location rather than the newly allocated one.
The solution I use is to keep resetting the router until a "genuine original UK" IP address appears. It's pot luck each time and can take me between 10 and 20 resets .