What should the provider refund if I use my right of redrawal?

Am I entitled to a refund of my remaining data allowance if I switch broadband provider? What happens if the contract is terminated? What’s the difference between the right of withdrawal and open purchase? How do I cancel the contract if I use my right of redrawal? A company that charged me for a premium rate service has offered to refund the amount. How will the refund be made? Refund of unused call or data allowance