In law, there is no requirement for any form of negotiation or consultation before trustees make changes to a pension scheme. They are only required to consult with the company (not the unions).
In the case of BA, both negotiation and consultation took place with the unions. However, there was always the possibility of the Trustees (or indeed the Pension Regulator) imposing a deal on the staff.
Both the unions and BA were bright enough to realise that this was not the best idea so they entered into negotiations, which resulted in the agreement between BA and the Trustees last week. As a result, the pension deal has been finalised. Legally, the unions get no input on this.
Most unions have balloted their members to gain acceptance or otherwise (BASSA didn't ballot on this, strangely!!). However, the Trustees are required by law to protect existing pensions as their priority, almost to the exclusion of all else so the ballots themselves are effectively a rubber stamping exercise for what the unions have negotiated (or not in the case of the GMB). The problem the GMB now have is that further negotiations regarding the pension cannot achieve anything as the final deal has been signed off by the Trustees, which to reiterate, are not required to deal with unions. A strike will also have no bearing on the new pension deal for the same reason.
That may or may not be good news for you but it's the story so far....