I think this situation actually hurts up-selling. They are asking people to switch from discount ET (where a ticket can be changed for 30GBP) to discount CE where it can't.

I appreciate that late changes to CE cabin loads can be far more disruptive and costly to BA than late ET changes, however that is only a case for asking for a higher change fee, or a significantly higher fee within x-hours..

My only guess as to why this is the case relates to the mass of competition the ET cabin faces from other airlines forcing them to finally add a change option to discount ET (it was crazy not offering one and claiming to be full service) and that this competition situation is not the case with CE. While that may be true, it does reduce upselling which is something that (providing it is done at a fair price) has got to BAs interest to increase.

This lack of flexibility (i.e not for any price) is a big thing to ask people to give up when asking them for more money. Indeed I know people who've decided against CE becuase they wouldn't have any change option should circumstances radically change, they would however have been willing to accept a 50/60 GBP change fee. And I doubt such a proposition would damage flexiblie CE sales either.