What to do about gap in bottom of plasterboard wall when fitting floating floor (UK)

I have removed the skirting boards in my new build house, as I am going to fit engineered wood floor via the floating method.

The issue I have is that after removing the skirting, I was presented with these cavities at the base of the wall. They are around 2 inches wide, but vary in size around the room.

On the exterior walls there is also a gap in the flooring at the back of the cavity.

The interior walls have do not have this gap.

In addition to the below questions I was wondering the purpose of this gap?

I'm not sure how to proceed with the fitting of the floor though.

• Should I just lay underlay and leave the cavities as they are?
  Would this not allow a draft under the skirting?
  Would it allow too much movement of the floor?

• Fill the cavity completely?
  Would this affect any insulation/damp-proofing on exterior walls?
  What should the cavity be filled with?

• Fill the cavity to within half inch of floor?
  This is how the walls were when I have previously laid flooring How would I go about filling to this level?

Appreciate any assistance with this and happy to provide any more information if required.