Embarrassing! My only excuse is that Aug 3 is precisely when I left for this trip, but my review of forum archives certainly should have found those threads.
Well, I can personally confirm that this new policy is in full effect, and LHR staff are enforcing it diligently.
Do FT readers think there's any duty on AA to inform pax of its decision to collect this tax, in advance of AA starting to collect the tax? If AA knew all along that it would collect this tax beginning Aug 3, AA could reasonably alert the EXP desk (who surely receive and process most VIP upgrades) to in turn alert EXP pax who may not have expected the fee.