Battens have the advantage that you can level them at your leisure so that the plasterboard fixing is straightforward. Celotex between them loses the same 50mm as dot dab. If you want to maximise the foam area, use 2"x 1" battens. You only need 2" x 2" where 2 boards meet
If you butt the celotex boards up tight, then you don't need 2x2 anywhere - just use 2x1 and screw through the batten, and the foam into the wall behind. That way there is no thermal bridge. The foam sheets are plenty rigid enough.
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