I did this last year. I called to cancel. Avios redeposited to my account. Taxes refunded minus the $55 pp fee. Only took about 10 days. Quite simple. However, the avios you received for the taxes will not be deducted from your account. They will offset any new avios for taxes in the future you might receive for future taxes. I paid $1500 in taxes originally and got 4500 in avios for the taxes. When I cancelled , they didn't deduct the 4500 avios from my account, but it like a running total. If you pay 1600 dollars for a future flight, you will only receive 300 avios on your taxes.