I got mine from Costco two years ago £4,5k installed and for the same can it was £7k elsewhere and ive been very happy with it, its 44mm and kitted out as a home office/ man cave.
Heating wise I have a Dyson hot and cold fan and set it at 16c at all times and the when im going up there I just turn it up accordingly and its been fine.
this is the one I have, I laid the concrete base and they fitted it in 2 days
https://www.costco.co.uk/Garden-Shed...-42-m/p/237946