When you ring up to tell them that you want to leave, you'll get put through to the "retention" department - it's their job to try and keep you and persuade you NOT to leave .... they have more power than the regular sales staff, as in they can offer you bigger discounts.
When my contract expired with Vodafone, I knew what phone I wanted, told them I'd found it cheaper elsewhere and I wanted to leave. Vodafone's advertised price for the phone bundle was £35 pm .... I got it for £24.15 pm with Vodafone .... result :)