I believe you are taking a reasonable approach, the only question I would ask is whether the drawings and/or rebar schedules detailed the couplers. If so then the Subcontractor had the information to be able to price.
The wider question it raises is if the MoM does not prescribe couplers to be separately itemised then how do you measure them since there will be no measurement rules!
The wider question it raises is if the MoM does not prescribe couplers to be separately itemised then how do you measure them since there will be no measurement rules!