O2 Nexus 4 is locked!

Just went to my local O2 shop to pick up a pay as you go Nexus 4 with the idea of using a three sim. I'd heard a lot of people saying these are unlocked from O2 so was very disappointed to discover that the store assistants persist claims that its actually locked were correct.

I got him to test my mates three sim, didn't work, then my tesco sim, which didn't work either. Very annoying!