No concrete infill to cavity wall below ground level

This is the reason there is a 200mm void to stop sitting water getting to your internal floor

You don't have cavity below ground it will fill with water and the ground pressure could collapse the block work It will need filling one way or another

To address water penetration in the bottom of the cavity wall, you might consider tanking the outside wall. Applying a waterproofing membrane or tanking solution can help prevent water ingress and protect the foundation from further damage. Additionally, improving drainage around the area could be beneficial in managing water flow away from the foundation. Consulting with a structural engineer or a professional in construction would provide a more tailored solution based on the specific conditions of your extension.

The cavity fill should be the same level as ground level to stop pressure from the side walls collapsing, and filling with groundwater, which could freeze and blow Brick Work should 225ml below dpc