The space looks a bit cold & clinical at the moment. Add sheer curtains to the window from ceiling to floor this wall Add some drama and also help to soften the look. I don't think the corrugated iron works use timber instead . A rug will better define the space perhaps with pops of colour like yellow , orange or red this will add some warmth. Prints on the wall , large pot plants side tables and floor lamps . In the billiard area it would look good to add a pendant lamp over the table you can buy ones specifically for billiard tables . More pops of colour also a couple of occasional chairs . You could also add more timber here to give more ambiance. Happy decorating .

You never mentioned your budget that you're willing to spend. You could add a thicker fill backed insulation blanket - 50mm, 75mm or 100mm are the common thicknesses. Steep roof pitch so would need scaffolding, harnesses, etc.. which adds up quickly. Probably still not enough insulation though but probably cheapest option. Insulation under existing timber linings then reline over the top of that as some have suggested. Working at height though so would need scaffold set up again and working overhead is not fun nor easy. You could replace the roof with a sandwich panel roof. Try Versiclad or Kingspan or comparable. They have a polystyrene type of insulation between an outer metal roof sheet and a metal inner sheet. These are heat laminated together. Again different thickness available from 50mm up to 180mm is common. The 180mm has a R rating of 4.7 which is quite good. Again steep pitch, etc., probably need new ridge capping, etc.. Probably most expensive option...... Keep us all posted on your decision though!

sounds excellent: i love corrugated steel with brick and fibro panels but consider applying cladding vertically so dust doesn't settle on corrugations and look dirty all the time and check out some of the project homes builders eg GJGardner for numerous examples of single storey homes with mixed cladding and here are a few examples off HOUZZ

You would have to ask SCA. I believe the gable roofs are for council (streets-cape). The cladding being corrugated, cost (pre-finished) and proximity to the coast (durability).