Snap, CC, they've increased mine too!
While the Bank of England base rate is at an all time low (1%), much of the money the British government provided to the banks, to help them through their difficulties, was effectively in the form of a loan with an interest rate of 12%. So the banks are having to add their profit margin onto 12%, rather than 1%.
Further, the moves taken by governments throughout the world to inject liquidity into the money markets, have only been partially effective. Banks are still finding it hard to borrow money from other institutions; when they can do so it's often at high rates, which they then pass on to borrowers.
Lastly, the recession means that more people are likely to find that they can't meet their financial obligations, and will default on their debt repayments. So the banks know that more of their loans will 'go bad' and they have to charge their card holders extra to cover the losses that they know they'll suffer.
Chris