If it's the case that 50% of travellers are connecting as quoted upthread, then introducing APD to connecting pax would mean being able to halve the rates while still bringing in the same revenue, resulting in a fairer system which doesn't penalise people for actually stopping over in the country. SO it would be £125 for the longest flights in premium cabins, and £50 in economy wouldn't be much more than the German equivalent, which does apply to connections iirc. That and reducing the rates for ex-UK travellers will encourage more people to book into WTP in particular, or just take more flights by lowering economy prices by £50 on long haul routes, so that would offset some if not all of the lost revenue due to lost connecting pax.