the IAA does very well, thank you very much, with fees from all the overflying traffic between the UK, Europe and North America every day.
ENROUTE charges covering costs elsewhere is not a runner for much longer.
IAA is a commercial entity so if the numbers are not there??
Remember that the CAR instructed the IAA to reduce its landing charges.
If the DAA want to subsidize the IAA so be it, if not then savings must be made.