If it was a buisness hire purchase agreement, it's very difficult to get out of as you cannot use the half rule. You'll just have to try to sell the vehicle privately and pay off the leasing agreement.
If you purchased as a private individual/consumer and have paid back half or more than half of the value of the vehicle, you can return the vehicle to the leasing company under the 'half rule' arrangement with no futher payments due. Do not , however, sign a 'voluntary surrender' form or you will then be liable for futher payments.
Talk to Citizen's advice or MABS and they will explain in more detail.