I think it depends on the starting price.
I would pay a premium from a £400 cheapie starting price in CE I wouldn't pay a premium from the thick end of £1k+.
I don't think it matters whether an odd few here would pay a premium what matters is the load factor comparison between the midhaul product and the shorthaul product (including the loss of Y seats to allow for midhaul).
I don't see how you ever build a case for this on most shorthaul routes. The one exception I can see would be to connect to HEL and increase code sharing or form a joint business on some routes with AY via HEL so that there is a real all J option through HEL.