That looks very like my local supplier, Austen Knapman. All I can say is that their online prices are always higher than you pay if you just walk in or order over the phone for collection. Saves maybe 20-30% on web prices. I last bought box section from them a few years ago when I built my router, and that was only 50x50x3, so no idea how that matches the going rate.

You are a long way from South Devon, though...