You can get Sanatogen brand protein powder, which has negligible carbs in it, from Boots or other chemists. Sometimes they actually have it displayed on the shelves, but most often you need to get them to order it for you. This is what I do - my local Lloyds Chemist gets it for me. You might have to take 4 or so packs of it, but it's got a long shelf life, so it doesn't matter. This is the brand I usually use for the recipes that call for 'unflavoured protein powder' in my Low Carb / Low GI Cookbook (see www.lowcarbiseasy.com/cookbook.htm).
i got some in the health food shop attached to my local Tesco. the guys in there were really helpful, as I was standing in front of hundreds of packets and tubs looking like a startled bunny!! It was all a bit scary but the guy in the shop was able to help.