have been using tesco, as its cheaper, was using bp ultimate. but to be honest they're fairly similar, and most fuels come from the same place.
there are only a handful of petroluem distribution centres, which supply all the retailers. so they all use the same fuel. its just tesco sell theres cheaper as they offset the cost through the fact that if they get you in the tesco centre with low fuel ,you'll probably spend money in the store while your there.
tesco maywell add there own additives, but additives are better than none regardless of how accuratly they are measured in. as long as its not full of crap, and won't clog my fuel filters i'll be happy.