I’m not lookin to be cheap as the roof is costing loads to be strong enough to support a sedum roof so lots of steel and joists to make it strong enough. I am looking at the bits that I may be able to do that i can save some money to spend elsewhere on the project. I fit were to live in it would be opening skylights but as its only for the cars I’m going for cheaper ebay ones so the money saved can be spent on a resin floor. It’s the same with the ceiling, I’m fine screwing things to the joists but plastering a ceiling is beyond my physical ability so its the labour thats the big cost. I have looked at various sheet materials but haven’t found anything so looks like board and plaster it will be. The idea of champhered edge board could be a good option similar to some good looking ply internal cladding I have seen on some builds.
As for artex, I will take that with a pinch of salt. That stuf is a magnet for dust and would look like it hasn’t been cleaned since the seventies after about a week.