Hello GHG, the problem we have is that the rules (in this case the Great Britain National Reference Sheet) clearly state that the Dutch colonies only become controlled or aligned to the British at the end of the combat phase :
“DUTCH COLONIES If Axis or Comintern declares war on the Netherlands, Netherlands becomes Controlled by or Aligned to Great Britain at the end of the Combat Phase. Once Aligned, it may distribute this income as it sees fit to itself, FEC or ANZAC.”
In your answer, you mention “As soon as the Dutch are are declared war on they come under the control of the British”, so that could be during a combat move against the Dutch and not only “at the end of the Combat Phase”.
However, if submerging subs is considered an integral part of the decision process of rolling and choosing casualties, then I agree with you.