Unless CPW have specific trial periods or cooling off periods for mobile broadband then normal Distance Selling Regulations would apply. You can cancel before you actually start using the service but once you start to use the service you are deemed to have accepted the service -
"services that begin, by agreement, before the end of the cancellation period providing the supplier has informed the consumer before the conclusion of the contract, in writing or another durable medium, that he will not be able to cancel once performance of the services has begun with his agreement" http://www.oft.gov.uk/advice_and_resources/resource_base/legal/distance-selling-regulations/regulation-exceptions
CPW Ts & Cs - If you are a private consumer you may cancel any purchase at any time within 7 working days from the day after completing your order for the Services (in respect of Services) or 7 working days from the day after receipt of the Goods (in respect of Goods) (“Trial Period”) without incurring any obligation or liability to us, however, you may not cancel any services once we have started to provide them with your agreement.