Originally Posted by FrancisA

Upgrading from WT to WT+ requires that you have a WT ticket in fare classes Y, B or H (read flexible and expensive). For that reason WT to WT+ (and ET to CE) upgrades using avios are not good value - a WT+ (or CE) ticket can often be purchased for less than the cost of the flexible WT (ET) ticket.

All WT+ and CW fares can be upgraded to the next cabin (with the exception of some sale fares). WT+ to CW is the best bargain as it requires only half the standard redemption amount to upgrade a sector.

If you have a work policy that books you into lowest cost WT (and you don't need flexible tickets) then upgrades to WT+ are not going to be possible. In that case I would use the avios for leisure travel either short-haul (as reward saver is very good value) or for long-haul holidays if you can book WT+ and upgrade to CW.