I read this on the Which site
If the contract wasn't made in person
If you buy something over the phone or the internet, or by mail or fax order, the Distance Selling Regulations give you a cooling-off period of seven working days during which you can cancel the contract.
If you're buying goods, the seven-day cooling-off period starts the day after receipt of the goods.
If you're buying services, the cooling-off period starts the day after you agree to go ahead with the service. If you agree with the service provider that the service will start straight away, you waive these cooling-off rights.
The regulations don't cover:
financial services,
holiday or travel arrangements,
tailor-made items or perishable goods, such as groceries, or
CDs, DVDs or software if you've broken the seal on the wrapping.
This would appear to fit your circumstances