2. internal cavity wall (later to be plastered)

Yes.

3. external cavity wall (later to rendered).

Yes.

P.S. For further recommendations, use Celcon Standard in the outer leaf and
Celcon Solar in the inner leaf. However, you'll probably just end up at the
builder's merchants where they haven't heard of anything except bog standard
3.5N 440x215x100 and just have to work these into your original plan for
foundation, inner and outer...

Christian.