Someone I spoke to was told by the one of the Balpa cc that they wouldn't countenance a w.t.r as it would potentially unfair to the rest of the crew e.g. non unionised flight deck or the cabin crew if you put something tech down route and have to nightstop, made everyone late, etc etc.
It would seem it's straight to ACAS, or fix bayonets... assuming it's a "no", which is FAR from a foregone conclusion