What Gove says isn't quite right - though I can see why the govt might have considered it safer to use existing public health powers as the Civil Contingencies Act requires that existing legislation *cannot* be relied upon without the risk of serious delay /1 
I think there was probably a concern that although it was possible Public health act didn't quite grant the powers which were needed for the lockdown, use of the Civil Contingencies Act could have been criticised because the public health act is designed for this situation! /2
So govt was caught between a rock (Civil Contingencies Act) and a hard place (Public Health Act). But none of that explains why it didn't just use the legislation going through parliament - the Coronavirus Act! Answer may be that no need for new legislation... /3
Clarification here
