I don't know where you are, but in most town centres in the UK there's a mobile phone repair shop, and second hand sales shop who can unlock devices for around ?10. I'm in Brighton and have had no problems in using such services. It's also not illegal to do (unless it's a stolen phone of course).
If you're leaving 3, or upgrading, then the carrier usually doesn't have a problem in providing the unlock codes directly to you.
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