The big question for me right now is this. Given that the
bedrooms and shower room will be directly above the room
heated by the woodburner, the stairs will be open and the
kitchen addition will open directly onto it. Do I actually
need radiators?

I'd go with radiators. I would also abandon the electric shower idea and go
with a heatbank. Mains hot water fed from the solid fuel boiler. As well as
an electric immersion, you can add an oil/gas boiler as well (even solar
panels) and just run the cheapest fuel.

Check your proposed layout for fire safety regulations. Having a kitchen
positioned on the main exit route from upstairs may be frowned upon, as a
fire in the most likely location may result in tenant toast.

Christian.