If I was a Cardiff management bod then I'd ........
Establish an airspace volume with Brussels CFMU for the airspace concerned. Agree routes, levels, etc, to be captured, thus allowing Brussels to create the 'environment'.
Declare a traffic capacity (monitor value or Target Sector Flow).
Insist the trainers file IFR plans through IFPS (assuming they don't already).
Have LFMP monitor the capacity versus demand, and if capacity exceeded, put on a Regulation with agreement of Cardiff or allow Cardiff to take other tactical measures to manage traffic efficiently (this can
also mean taking extra aircraft or improving slots, as well as restricting them by things such as MDIs). If there's Regulation delays, then that is because the airspace is too busy. The trainers will get their share of airspace slots in this system like everyone else. Whether that matches their airport authority slot is not really a Flow Management problem, it's one for the pilot to resolve. The airspace slot is part of an ATC clearance and is the one which overrides any other arrangement.
Issue ATC clearances, approach times, etc in accordance with MATS Part 1 priorities. Note that this may mean joining training aircraft holding outside CAS until workload permits their acceptance. Or holding en route to facilitate Normal category flight sequencing. A CFMU slot is never a guarantee of an unhindered flight path.
Any IFR trainers calling 'cold' should be accomodated as workload permits. If they didn't file a plan through the correct channels, then they will have to take their chances.
Errr, that's it
