Its always been the case that this was against the rules. It seems that things have been getting out of hand and so the rules have been restated.
This message has not been sent out universally but targeted, rumour has it that unofficial upgrades for mates have been for sale amongst certain staff groups. Certainly there have been some very low priority staff in F and J cabins over the last few months, with other staff with F & J entitlement stuck down the back. This has always gone on particularly at certain outstations, it seems BA are clamping down again - its cheaper to sack someone than make them redundant!!
It is still BA policy to upgrade staff before commercials as this protects the yield. Tactically FMU may choose to upgrade you to a cabin you do not have an entitlement to in order to sell that seat to a commercial pax.
Given the current industrial climate at BA it would be a brave member of staff who would flout the rules to look after a mate. It could be a quick and strange way to hand in your notice.