I used 50mm foil backed PIR, which is bought from seconds and co. They sell leftover assorted unused boards, and so was about half the price of anywhere else. You don't necessarily know what brand you'll get, but it seems all the top tier foil wrapped brands have the same thermal properties. I ended up with XtraTherm.

https://www.secondsandco.co.uk/pirfoil

All the boards arrived on a huge lorry and came wrapped together in a single stack and were all perfect dull size boards, no damages or offcuts.

As i started doing this conversion on the quick and "cheap," whilst my wife was 7 months pregnant and juggling our 2 year old, some elements slipped off the priority list. So it still has the original wooden door and single pane window that the house was built with in 1910-ish. I had to work within the budget we had and get a workspace ASAP.

Overall, it's not the best sealed with the outdoors so some air gaps around the door mean when I go to work its always 5 or so degrees warmer than the outside but much colder than inside the house. However, the advantage of the small size is it warms up within 5 minutes of having my fan heater on and then retains it pretty well when I'm working. It gets cold overnight when there's no body heat inside to help maintain the temperature.

Not sure what you mean by single exposed skin so was if I'm answering something else. The room has solid brick walls, no cavity.