They are locked by region (Geoblocked). Similar to how you didn't used to be able to use US DVDs in the UK or or Super Famicom games on the SNES.
Get yourself a VPN and set it to UK and you may get access from anywhere in the world although from what I understand, Netflix have recently cracked down on this practice so if they recognise the IP as belonging to a VPN, it won't let you access anyway.