how to get cables through cavity wall

I have a situation where the fuse box and service entry is located within a very small cabinet built into the outer leaf of what originally was an external cavity wall to a house. A garage was subsequently added on to the side of the house making the external cavity wall now an internal one. I have the job of installing a new consumer unit and would make sense to locate it on the cavity wall near to the original cabinet now inside the garage. I need to run cables from the consumer unit though the cavity wall and into the house. Problem is that the cavity is filled with polystyrene balls as insulation and I am aware of the plasticity probs this can cause. I would also need to run the cables vertically for about 0.5m to get from the consumer unit up to the level of the first floor void as the floor is higher than the flat garage roof.

What would be your views please of how to get the circuits from the consumer unit through the cavity wall and into the house for distribution? Obviously I would prefer to avoid heavy derating factors.

Surface trunking in the Garage to the level of the upper floor void, then cut a 4" hole in the wall, insert a section of 4" soil pipe to act as a conduit through the wall.

Ensure the Poly balls are blown out of the pipe.

Once this is done you can run cables through it quite easily. Once all the cables are in, ensure you fill the pipe with fire retardant foam as a fire break

Thanks FLW.

Good idea about the soil pipe and fire foam.

Problem is complicated by the upper floor void being 150mm higher than the garage ceiling. So still need to get 6" higher. 4" hole would come out just below the ceiling of the room inside the house. Any ideas please how to get from here into the floor void neatly to finish the job. Would you recommend cutting the 4" hole at 45 degree angle to get from top of garage wall through the cavity and into the higher void?

If you use Galvanised trunking to raise the cables up, this can go externally and be sealed using silicone sealent.

Reduce the hole size to accept 75mm or even smaller soil pipe, just ensure the pipe is wholly inside the trunking.