Morning Desertia
The contract actually says "and as amended from time to time." not as you said in your post "can be amended from time to time."
Again the whole crux of the argument is as to whether the CA's form part of the contract of employment - and as it currently stands those CA's have not actually been amended - BA has totally disregarded them and imposed their own agreement thereby breaching their own contract by imposing terms outside of the CA's.