I wish I had seen your post before in regards to the slate although the pick up of them might've been an issue, but 90p for high quality reclaimed welsh slate seems fantastic value.
To the 1st poster when I needed slate I just settled for looking online and ended up getting slate from here https://www.scotiaroofingsuppliesltd.co.uk/tiles-and-slates - Wont be that cheap however was happy enough with price and slate quality however if knew about the facebook marketplace I would've and will use that from now on myself.
Don't know what type of cladding you are looking for but when I recently got some I found that most of the quotes I got were quite expensive, and the companies that came in cheaper usually made it back with their delivery costs. Anyway ended up ordering cedar cladding from this company here https://dalply.co.uk/timber-cladding/ again happy enough with everything however I now also wish that I went looking for cladding on the facebook marketplace!
Hope that helps and good luck with the extension!