I have a BA codeshare on JL connecting to domestic JL flights and returning on BA, travelling in Y. BA allocated me a seat on the JL codeshare when I booked, but I wondered whether I could change it and also whether I could get seats on the domestic flights.
Checkmytrip.com did not have the JL locator so I phoned BA who gave it to me, but on the JL site it only showed the BA metal flghts! I phoned JL who said that it was the only locator and that they could not make any changes to bookings made by BA.
So no change since the last time I flew on JL a couple of years ago.