UK food and supplement laws are going to be unfamiliar to me, here in the USA.
But they should all list the ingredients, and a brand like Optimum Nutrition should be available over there.
Most 100% pure whey protein powders won't have anything special in them, like the 'testosterone' boosters (which don't work, by the way, so you're smart to avoid that hype). Just protein, a couple grams of carbs and maybe a gram of fat.