In case anyone wants to be able to move the enemy castbar in the default UI, I have found an addon called ‘Moveit’ that does this.
First time /mit to open the options and in Add Frame type ‘TargetFrameSpellBar’ (case-sensitive), then click Add Frame.
Once the frame has been added, you can move the X/Y or change the scale etc.
The best way to move it and see where it is (as it seems to not be where the blue bar shows) is to select yourself and use your hearthstone
Anyway, hope this helps someone