How do I bathroom / kitchen / extension? etc.

I think I might have found a better location - in a large coat cupboard under the stairs.
It'd still be on the same internal wall but there is already a boxed in section across the ceiling of the hall and through the toilet opposite. The run would be approx 3m, although I believe it's now possible to use carbon monoxide detectors instead of inspection hatches in the void.

In a worst case scenario I think I could house it in a wall unit on the external wall of the kitchen, so long as I can run the water and gas horizontally from underneath into the corner of the room. I don't want to have to box in the pipes between the underside of the unit and the worktop.