Since you're in the UK, what about giving A&A Crack & Sons a call? They supply leather for a lot of shoemakers. I'm sure they would have some suggestions for thick leather suitable for uppers.
If you're still looking, this italian dealer carries Badalassi Pueblo:
https://buyleatheronline.com/en/badalassi-carlo/400-badalassi-pueblo-veg-tan-shoulder.html
In fact I just got a beautiful piece of Pueblo from them two days ago.