Possibly because a ticket re-issue is needed and has not yet been done. Anyone who needs to pay to pre-allocate a seat can't do so until there's a valid e-ticket coupon for that sector.

If you want to dig deeper into this question, then go to classic.checkmytrip.com. Put in your locator and surname to pull up the booking.

On the first page that you see, you should be able to see your booking and the cabin for each sector. This ought to say confirmed for Club on the outbound and WT+ (or premium economy) for the inbound.

If there's an "e-ticket receipt" tab, click that. You should see a box or a set of boxes for each passenger. If (for each passenger) you can see only a box that refers to BGI-LGW in WT+ (or premium economy) and with a fare basis starting with the letter T, then it means that your ticket still needs to be reissued to reflect the upgrade that has taken place on the LGW-BGI sector. Until that re-issue has taken place, you won't be able to pay to pre-allocate a seat.

In addition, until the ticket has been re-issued for that sector, you won't be able to see the fare basis for that sector. Until you can, it will probably still remain a mystery as to why you've been upgraded into I class, which is a cash booking class. IME these are not normally used when complimentary upgrades are given; it's more usual to see them booked into U class (which is also used for awards).

So is there any possibility that a kind fairy has been along to pay for a cash upgrade for you (or as a birthday present for your friend)? It's nice if someone has done it as a surprise, although if that's what's happened it will have had an adverse effect on the flexibility of the ticket that you will now have.