Originally Posted by Globaliser

What's ridiculous about it? As I read your post, £99 was the price (presumably a special offer aka POUG price) for upgrading LHR-CDG.

That would not involve any promise that it would also cost £99 to upgrade CDG-LHR. There may have been no special offer available for that sector, and it may have been that your only option was to upgrade in accordance with the fare rules to which you agreed when you bought your ticket.

You're never entitled to a cheapie upgrade. You can only get them if BA has some reason for offering them.