On TripAdvisor, you can choose your country from the little flag in the top right-hand corner; clicking on the flag that is on display will bring up a menu of flags.

This works while you're doing the searching; when it comes to paying, or even requesting prices, the site may revert back to that of the country that you're in (or that the site thinks you're in, or ought to be in...). When this happens, it ought to work if you alter the url manually in the address bar; so, if you're at <...tripadvisor.de[blah-blah-blah]> and you want the UK site, change it to <...tripadvisor.co.uk[blah-blah-blah]>, leaving the "blah-blah-blah" part unchanged. This ought to allow you to reserve and pay using the UK site.

This sort of thing is not unique to TripAdvisor. Why do websites that do this? Do they think that they are providing a service to their users? Or is there some perceived advantage to themselves?