I don't think we should read too much into Unite taking professional legal advice on the matter. Their members jobs are on the line if a further strike is called, and they must be aware of it.
If Unite's QC advises that in his/her view, BA's responses to the points covered in the letter could be sucessfully challenged in Court, then perhaps that will be the route Unite takes rather than risking their members jobs in another (what will almost certainly be) fruitless strike.
Such a cautious approach may well upset the more rabid pro-strikers though.