Don't forget NRT and LHR have many government restrictions.
For PHL-NRT, you would have to get the Japanese and US gov't on board, and deal with other airline's objections. Adding a new NRT service is difficult. Not only are there gov't route authority issues, but NRT slot constraints as well.
For LHR, I don't think U would be able to code share on LHR flights. UA and AA are the only US carriers permitted to operate into LHR and I doubt another US carrier could just attach their flight numbers to LHR flights that easily. You would have VS and AA in a tizzy, as well as DL, NW, and CO, who have been wanting LHR access in some form or another for years. I don't know all the details of Bermuda II so maybe this would be possible, but I kinda doubt it.