If you have volunteered to surrender your reservation on an overbooked flight, this must be in exchange for benefits under conditions to be agreed between you and the carrier. This might be in the form of vouchers.

In addition you are entitled to;

A refund within seven days of the full price you paid for your ticket  for the part or parts of your journey not made (and for the part or parts already made if the flight is no longer serving any purpose in relation to the your original travel plan) and a return flight to the departure airport as soon as possible.

or

Re-routing to your final destination as soon as possible or, if you agree, at a later date. (If the airline flies you to another airport in your destination city then they must pay for the transfer to the airport you were booked for or to another close-by point of your choice)