Neither it what they give out when you get a contract with them, but they sell this as sim free "Not Unlocked" at the same price as what you pay if you was to buy a fully unlocked one from Apple.
Like I mentioned it will lock to the first sim you insert, 02 are pretty good when it comes to unlocking, but the other like Three, Vodafone & EE might make life abit hard to get it unlocked, because you haven't actually bought it on either contract or PAYG from them.
It the risk you take if you plan on selling it on.
here the link
http://www.carphonewarehouse.com/mobiles/mobile-phones/APPLE_IPHONE_5S_32GB/HANDSET?colourCode=GREY