I heard the suspect fuel came from Essex, but has been distributed all over the place. My local tesco had some apparently, which is in Byfleet, Surrey.
Personally I tend to use super unleaded, or V-power, generally the best that is available at the time, and rarely use supermarkets.
I do notice a slight difference in performance, though it may just be in my mind.
I've been using it since my days as a 16 year old on a moped. It was the difference between 30 and 35 mph back then, which to a 16 year old is a lot!

It also made a huge difference on my 125cc, considering the cost difference isn't too bad on a bike, I just use it anyway now, probably better for the bike in the long run.