Are you really suggesting that BASSA should be negotiating with the TAX man?
As the an organisation that is paid to represent the interests of all of its members, yes.
But it doesn't have to do so in isolation.
Did BASSA use professional advisors? Yes they cost money. Though the cost would probably have amounted to at most few quid per member, which given how much as been shown to be at stake, would have been a worthwhile investment and probably a better use of money than hiring out racecourse venues and pursuing infinitely more expensive litigation in the courts. Professional advisors have lots of experience with negotiating with HMRC and can see "the bigger picture" (ie what HMRC have done elsewhere).
How well did BASSA work with BA management? BA will have a sizeable in-house tax department with professionally qualified accountants/tax advisors who again have existing relationships with HMRC officers and lots of experience with negotiating with them.
No-one expects any rep to be an expert in tax. That's not what CC are paid to do and a basic rule I was taught in my job (as I said before I have a lot of professional experience working with HMRC) is if you don't feel you have the experience/expertise to deal with something yourself, bring in someone else who does - that is a sign of strength not weakness.