The number of UK air transport accidents involving aircrew detected with illegal alcohol levels for the last five years is zero.)
It is hardly surprising that there have been no incidents of aircrew being detected with illegal alcohol levels. Prior to the Act there were no illegal limits and no powers to test for them.
Either the CAA reply is poorly phrased or they are being disingenuous. I also note that they do not say whether the limits were set in accordance with their advice. I do not imagine that DfT would have acted contrary to advice from the CAA in this matter.