I can’t think that any airport in this country has engaged in fraudulent tax evasion as you seem to be implying. The relevant authorities would take action but they don't seem to have done so which suggests no fraud or other illegality. I'm certainly not aware of any reported instances in the UK.
It was reported in the press that at least one airport in the private sector not a million miles from CWL avoided paying corporation tax in the past as it was able to use its operational profits to help to pay off loans to its shareholders.
Tax avoidance, as opposed to tax evasion, is not illegal although some might question its morality. Many organisations and individuals use the complexity of taxation laws to try to (legally) mitigate their taxation bills.
It's a safe bet to say that the Wales Assembly Government both as the government and as CWL's owners would have taken the best legal advice available to it before embarking on making the reported £13 million loan available for route development, and is confident that its actions are legal.