You have to check the fare rules as well. Currently on that flight that BA is willing to use fare bucket J C D and R but not I. When it comes to sale each fare bucket assigned to a fare rules. These fare rules can have different conditions depending on who sell it under which channel. So like public R fare rule needs a 90 day advance purchase or a Saturday stay, etc. It could be that a corporate R fare rule is not that strict so you can buy under that R fare bucket.
So in your example check what is the conditions for the R fare rules so you will know that you can buy or not. So ie if it has a 90 days rule then you won’t able to buy under that fare condition only the current D fare, etc. If you wait it can be worse because as the D fare can have other restriction which could prevent you to buy that fare and only C and J left.