I'm sure the last time they increased prices we got a letter saying you were entitled to cancel your contract (even if you were only a few weeks into an 18 month deal) as long as you did so within 30 days of the notification of the price increase.
Even if you have no intention of doing so, call them and threaten to leave because of the increase. They will usually offer you a discount (especially if you're already paying full price). But as sbcmfc says, do your homework before calling, as it will carry a lot more weight if you can say "I'm moving to x as they are offering me y package for £z".
The uswitch website is usually quite good for checking what deals the other providers are offering.