[tcbass]

Some Independent Mobile phone retailers will sell unlocked phones, depending on model/ network etc. it's worth asking what they got.
Vodafone has always unlocked my phones for free when I have reached the end of my contract (which I have always done regardless of my plans for the phone). The only other reason you would want to do it before the end of contract is (as said earlier) you want to use a foreign sim abroad or you want to give away/sell your pone, but you would still be liable for the contract (if you have one).