APD
Understood that airlines don't like LH tags or 3 legged routes but..
BRS/CWL are so close together and IF APD is devolved a routing of DOH/BRS/CWL/DOH would give QR or whoever a 2 way benefit.
Larger / wealthier catchment area of BRS coupled with the better
operational capability of CWL.
Think I'm correct in stating that APD is payable on departures not arrivals though quite how that would work out for departing BRS pax - not too sure.