I think that the best value comes from long haul trips in premium cabins. I look at the spot revenue ticket price and work out the value I'm getting for each Avios (Avio?) it costs me

For instance, I've just used an expiring 241 for an open jaw return to the US next month. The cash ticket price when I booked would have been around £7.3k (I booked around a month out, but the Avios redemption values don't change as long as there is availability?). After I deducted the 'taxes' from the revenue price and divided by the number of Avios it cost, my Avios were worth around 8.3p each.

Bearing in mind that I value each Avios I earn at around 1p each, I don't think that this is too bad! Another consideration is that I have access to tickets that I'd never dream of paying the full cash price for but, with availability, I could book it almost at the last minute.