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 .

If you want rusty corrugated just get some mild steel rolled by your corrugated supplier. galvanising is way too hard to remove and will actually cost more than rolling new mild steel sheet. If you want fast rust just give it a light grit blast to remove the mill scale. It is not really needed as it will get there after one winter anyway ( as long as it is in full weather.

Another idea i have seen in a few old timber houses, and that works really well is to move the kitchen from this large room entirely and create a more modern galley kitchen in a sleepout. You could consider fitting in a kitchen with a long bench where the archway is on that little annex room in your second picture. Or some other space entirely. Don't feel bound to use the rooms as they are currently being used. Old houses were often not originally designed with things like a kitchen and a bathroom and so yes they were often tacked on hodge podge and don't often work. Also we like more open plan so find the area of the house that will be easier to open up and is on the right side of the house to catch the light to create the lounge area. Even if that is opening up 2 bedrooms. I really don't think that matters if what you are working with is just barebones timber spaces. I think your colour scheme is a great choice. Its classic and will show off the bones of the home nicely. The trouble with these larger kitchen rooms is that they just aren't as functional, even though there is more floor space a lot of it is wasted. Sometimes you can end up with more functional storage and space by using a smaller room like a sleepout or enclosed verandah. You can use an awkward sleepout space more productively and gain an extra room when you remove the kitchen, and the added advantage is you can slowly build you new kitchen and still have somewhere to cook in the meantime. With the new kitchen built you remove the old!

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!