77WBoeing since LHR-BOS is your most significant sector (Crosses IATA Area 2-Area 1 boundary) the luggage rules for that flight govern your basic baggage allowance. This should be shown on your reservation.
To this you can add your OWE bonus of 1 piece.

Note that the piece concept applies to your entire journey since you are flying into IATA Area 1 (The Americas).