is it a one way or return flight?

If you booked one way and can fly with hand luggage only, I'd just get off in London - if return, I wouldn't want to risk them cancelling the rest of the ticket due to a missed sector.
(I have once successfully argued that the system must have screwed up when they cancelled my return flight from HKG based on missing a leg of the outbound journey - but I wouldn't want to rely on it. I had flexibility and knew there was rewards space to book in should they not let me on a plane)