Arctic
As for if the PIC can sit in the "front passenger seat" the flight manual should state which seat the PIC must sit in. For most aircraft it will be in the limitations section. That being said, I understand some of the newer Eurocopter models do not stipulate (EC120).
In Aus, there is an allowance to carry a passenger in a seat with fully functioning dual controls as long as they are briefed to basically "keep hands and feet off" by the PIC.
No doubt the UK CAA has some antiquated system in place but I don't know.
