So I asked some questions at the Burberry store & did some online research recently.
The Burberry Brit line is not 100% cotton & only water resistant, but not as much as the heritage or London line of 100% cotton gabardine. It won't work in heavy rain. The Prorsum line is runway & also only water resistant. Gabardine was invented by Thomas Burberry.
The cotton gabardine in the London line & heritage styles has something done to it when being woven to make it waterproof. All the ones I posted are heritage/London, 100% cotton & waterproof. There is double cotton in the Prorsum line but it is only water resistant.
There was a time when they produced poly cotton gabardine, & it wasn't necessarily they cheaper Burberry Brit, & maybe Brit wasn't even around then. I'm not sure how waterproof those older ones are. Also dry cleaning can reduce the water proof finish I have read, which bothers me with a light coloured coat, but I will ask Burberry to recommend a dry cleaner for me.
My concerns with ebay. I bought one on ebay & she allowed me to come & see it & get a refund due to a faint mark she found on it after I won it. It looked great & I really don't think she was trying to sell a fake but I am not sure that it wasn't one, after reading what to look for. It was well made, fit well, but it didn't have some of the things I was told to look for. I also don't know how much it has been drycleaned & how.