Originally Posted by Mr Blobby
Then I get a box that displays a red oblong with GB and arrows above and below.
That's the player's country code, not the Region Code, it determines what language you rplayer menus are displayed in, what audio and subtitles a disc will default to. Until very recently it had nothing to do with Region locking.
A new trick that some distributors have discovered is to get a Blu-ray disc to discover a player's country code, and disable playback if it's outside that country (you can get round it by resetting the country code from the set up screen but not all players allow that.)