Have you tried Google?
I find its a wonderful tool as there are many, many companies out there with Summerhouses for sale!
Well... Thats is the first thing I did, but found them not to suite what I am after, plus looking at some sites things looked great, but when reading reviews about some of the sites things didn't look to appealing with problems people had, but the waltons one was not thrown up in my google shop search and I have seem to have found two sheds/summerhouses that meet the criteria.
What are peoples experiences from building them from scratch, from what I can tell the sheds from stock look pretty good with 12mm wood.
Also is there any means of insulating a shed? Not to the extreme end where its warm in snow but thats its fairly warm and doesn't heat too much in the sun? Yes I have checked google but its not being to helpful for me, instead all its said is the shed needs to breath and keeps offering or pointing to professional services?