Hi. Within Star Alliance, the following booking codes are used for award flights X: Econ, I: Bus, and O: First. I have travelled on a mix of X, I, and O on the same business class award ticket. For example, I have travelled from ARN to ORD on SK in I, continued on a UA two class flight in First (O) then returned to ORD with a single class regional jet in econ (X). As BD service AMS-LHR is single cabin service, and you have paid business miles for the whole ticket I would be surprised if BD does not allow you access to the lounge. I have not had this issue on similar bookings within the alliance. Bring a copy of the booking with you.