Hi,
I know that certainly if you took the contract out over the phone, you have 28 days to cancel the contract, as per the Distance Selling Regulations Act.
As for in store, I'd suspect the store agent was lying... sorry I meant mistaken! Check out their refund, cancellations and returns policy on their website, print off a copy, and go into the store armed with it. If they try anything just say you'll be lodging a complaint with Ofcom.