There are some experiences almost directly above! It is landside but near enough foolproof and perhaps would allow the briefest of lounge visits too.

Basically go through passports - try to be one of the first off, since the passport check process does take a bit longer than one would like. Then through the baggage hall, past the probably unstaffed customs desk, out to landside, bear leftwards towards the car hire desks, and then you will see a long sloping travelator which goes up to departures. When you get to departures, facing the check in desks, go right to the end of the building for security, which outside holiday season will be fairly swift. If you have lounge access it is on the same floor just after security, otherwise it's down again to the duty free shops, the Schengen border and the BA gate. LUX enforces fast track but the channel is sometimes hidden by the queue of people.