This has nothing to do with Html Agility Pack, it's because you have set AllowAutoRedirect to false. Remove it and it will work. The site apparently does a redirect, you need to follow it if you want the final HTML text.

Note the Html Agility Pack has a utility HtmlWeb class that can download file directly as an HmlDocument:

HtmlWeb web = new HtmlWeb();
HtmlDocument doc = web.Load(@"http://www.asos.com/ASOS/ASOS-Sweatshirt-With-Contrast-Ribs/Prod/pgeproduct.aspx?iid=2765751&cid=14368&sh=0&pge=0&pgesize=20&sort=-1&clr=Red");