Can the lighter 6kg airated breeze blocks be used for:

1. foundations

Usually, but check the specific product for suitability. Celcon Foundation
or similar is recommended for this application, but many standard blocks
should also suffice. However, a wide block like the Celcon Foundation
seriously reduces the amount of labour required, as you can lay the entire
thickness of foundation in a single course and only split into cavities
above the DPC level.

2. internal cavity wall (later to be plastered)

Yes.

3. external cavity wall (later to rendered).

Yes.

Christian.