You may need to book 2 separate tickets (SLC-BOS and BOS-LHR). There is a risk associated with that - in case of IROPS on the SLC-BOS portion you are not protected for the rest of the itin. Same thing on the return - if your LHR-BOS flight gets delayed and you miss your connection, you're SOL. DL may help you, but they certainly don't have to.