You can test it out on your manage my booking by clicking on cancel your flight. You can check it and get to the last page before confirming. I did this recently and was presented with 2 options:

1. Cancel the ticket and get a refund - in which case you would be subject to the cancellation fee as stated in your booking.
2. Cancel as a voucher - in this case, you would not be charged the cancellation fee and would receive a voucher +10%

I can't quite remember, but I don't think it's exactly 10% on the price you paid, it may be 10% on the pre-tax fare or something. Here is how it looks for me: