Personally, I wouldn't trust any medical diagnostic kit sold over the internet. They are usually in league with people who sell you "cures" as soon as they "diagnose" you with a problem.
I don't know about the UK, but over in the US we have allergy clinics and they are the ones that test you for allergies. I know some people on here have been and that they have been diagnosed with some intolerances. Basically they showed a mild reaction to something, but not a full-blown allergy reaction. I have heard it is a pretty imprecise science when it comes to foods (I know they nailed all of my mother's non-food allergies, though), but it can go a long ways towards confirming any suspicions you might have. In other words, if you think you have a problem with dairy, and it shows an intolerence to dairy proteins, then between your observations and the tests results, you can figure that you probably have a dairy intolerence. This means that when you consume dairy you may or may not react badly to it, depending on a number of conditions. It's my understanding, though, that if you have a true allergy, you have a bad reaction every time you come in contact with your irritant.