If you are with Virgin they will usually unlock their phones free after you have spent about £30 on calls. Other networks will usually unlock for a fee which is often less than the equipment or software advertised to unlock
If you have a Nokia DCT4 series (3300, 3510, 3510i, 3650, 5100, 6100, 6220, 6310, 6310i, 6500, 6510, 6610, 6650, 6800, 7210 ,7250, 7250i, 7650, 8310, 8910, 8910i & N-Gage) do not pay to get them unlocked. The people that unlock are rip off merchants.
You can download the unlocking software from click here and it will generate the cod, all you need is your IEMI and then select the network provider you are locked to :-)
Most local phone accessory shops charge a small fee of 10 pounds to unlock phones to all networks. They also offer a discount if purchasing say an alternative sim card or twin sim cover at the same time.