Yep you can buy a new phone. I found the K800i on their site was about £40 more expencive than it was on O2 but came with £40 of credit and a bundle of 800 messages* so I took that one. They also sent me the unlock code for it once I credited the free £40 to the account.
*not that you can use a second bundle on your account with the 300/300 bundle
