As I understand it, CAS determined that Etihad is not a related party? (An easy determination to make, since there are objective tests as to whether a party is related or not.)
If that is indeed the case, then Etihad as an unrelated party pays what it likes and that is by definition is fair value, i.e. A sponsorship deal is only worth what someone is prepared to pay for it, and provided they are an unrelated party, whatever they are prepared to pay, is fair value. We could get a sponsorship deal from Elon Musk for £10bn a year and that would be fair value, if Musk was daft enough to pay it.
Bottom line is, provided Etihad is deemed a non-related party, they can pay us what on earth they like and there is fuck all the PL or anyone else can do about it.