Can anyone answer the question as to the BA official Policy of offering discounted on-board upgrades when space is available?

Ie, is this only on selected routes (ie NA to UK), only when catering is available or is it totally at the discretion of the CSD for each flight as was implied yesterday?.

Like I said, WT to WT+ was stated as available yesterday, but not WT+ to CW although there was definitely space available in both classes.