On the LED route I am very sure it would come up, it's 150 pound upgrade pp one way. But I see above that you can buy for 382 outbound CE and inbound from DME in ET? So not sure ET return + 150 makes it cheaper, does it? If you can buy it immediately then of course better be safe.

The option comes up after a while on MMB, so doesnt matter whether booked on ba.com or on phone. Of course it is in their discretion to offer this so no guarantee from me, but that's my experience on 4 return flights since end november.