I am looking at the same thing, only JAX-ATL-BOS in Y and BOS-LHR-BOS in I, then BOS-ATL-JAX back in Y. I called and it was 1500.12 BOS-LHR-BOS in I, and when I added JAX to the mix, it was 1512.xx.
Just call and tell them you want Y for your domestic legs, and I for the TATL flight, and it should be cheaper than 2 separate tix.
YMMV.