Just a data point on converting the value of a cancelled flight to Avios. I got 226,100 avios for a flight costing £1,825.94. This was a booking just for me so the value was not split between multiple passengers which would have reduced the rate a little bit. So my O-level/GCSE maths makes that 0.81p a mile (rounding up). So I think that makes it the cheapest way to purchase (as opposed to earn) Avios if you are so inclined to top up your account.

I may well end up using these to make some GPR/Double Avios bookings for flights that are likely to be very expensive cash tickets (school holidays) so that low base value will help.