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.
Even if Wales had reduced APD to nil for flights departing the Principality a BRS-CWL-DOH would attract the APD rate in place for English airports for the entire journey in respect of those departing from BRS, ie long haul rate, as would a CWL/BRS/DOH for those departing from BRS.
So if Wales had secured devolved APD and reduced it to nil and the UK government maintained the present rate for English airports (currently £71 economy, £142 premium - rising to £73/£146 next April, although under 16s will be exempt from next March) the difference would be significant.
Many years ago airlines did operate CWL/BRS/ANOTHER or BRS/CWL/ANOTHER (both scheduled and charter) but this hasn't occurred much in the past 20 years.