What is legal or illegal depends upon the decision of a judge/s. until then it is what your lawyers interpret the likely outcome will be.
If a group of pilots chose to take BA to court and BA chose to fight it then it will take years to go through and cost a huge amount.
Im basing this on history which shows most people run out of money or the
risk v gain becomes too great.
BA may decide its not worth the hassle and acquiesce but if they don't and you don't have main BALPA support you will have to be unified and dig deep.