The Wales Assembly Government would have to be pretty dopey if they made a loan to themselves as owners of CWL in a way that was likely to breach state aid rules.
Even if the details of the loan were challengeable I wonder if the BRS owners/management would bother to pick this particular fight. Their own airport is increasing the momentum of its growth of the past five years and the several thousand passengers it might lose to CWL because of Flybe might be considered relatively negligible and not worth a legal battle that it might not win.
As has been pointed out, if a state abuses its position in providing state aid then it can be held accountable but so far as I'm aware there is no evidence that the WAG has done this.
From their public pronouncements the BRS management is more concerned about APD being devolved to Wales.
I don't understand bycrewlgw's point about BRS and state aid. What state aid does he/she think they received? Any incentives they might have given airlines would have been from the airport's own funds or from commercial borrowings which don't impact the state aid rules.