I used to have to use BA Staff Travel at T4 due to our airline's One World alliance. Never a good experience. Always came out feeling a bit smaller and less important. It is/was a bloody awful way to handle staff. A really badly pathed bureaucratic system dreamed up by some incompetant.
Now I can use a self-check in machine with everyone else, choose my seat , drop off my bags at the express desk and be off in a minute, domestic or international.