Several years ago, there was an UA operations to LHR from BOS.

Couldn't you take LH (with UA codeshare) from Boston to MUC or FRA? Now that LX is on *A, if UA codeshares with LX at this point, you could probably grab LX flights as well. I don't know much about UA/EK RTW ticket, but it would be much easier if you did *A RTW ticket.

If it must be on UA, then, like others mentioned, IAD or ORD are your two connecting points to LHR, AMS, MUC, FRA, CDG...