Are you (OP) under the impression that you're entitled to a ration of overhead space for whatever you wish to put there? Because I'm afraid you're not. If you're carrying only one smaller item, be it coat or bag, you should be prepared to put it under the seat in front of you.
Another testament to the superfluousness of the yellow tag brigade.
And yes, I know what a yellow tag 'actually' means. It's a very silly thing, and there's a reason BA isn't emulated much in this regard.