From the judgement, papagraph 8 (my bold):
The present position is that there are several collective agreements between BA and the relevant trade unions which appear to cover almost every aspect of the cabin crew working terms and conditions. None of these collective agreements is enforceable as between BA and the trade union in that in none of them is there any express intention recorded that the agreement should be enforceable, as is required by section 179 of the Trade Union and Labour Relations (Consolidation) Act 1992.
Oh dear!