Originally Posted by Globaliser
To be pedantic, it's only 10%, only off the base fare, only valid for leisure travel (though how that condition's enforced is anyone's guess), and only valid for ex-UK trips. You also have to be on the main UK register; US ADRs don't count, AFAIK. But it's valid for shareholder plus up to 5 (I think) other pax travelling with the shareholder.
Thanks Globaliser, couldn't be bothered looking it up in the BA sticky