locked to the 3 network but can be unlocked apparently.
a) It might be worth checking whether it's actually locked or not. I had a HTC One Red which was supposed to be locked to Vodafone, but wasn't. Try it with a different SIM card.
b) If it's definitely locked to 3, pay £3-4 to have it unlocked. There are plenty of services on eBay which do exactly this and they've worked perfectly for me. Before I had my One Red I had a One in silver which I got unlocked via eBay.
You'll make your phone a lot more tempting to people if you get it unlocked yourself rather than telling people that they're able to do it themselves
Awesome phone by the way - If I hadn't had to get an iPhone (family and extended family ALL have iPhones and we've got a baby on the way so photo sharing is much easier), then I'd still have my HTC