As Sid says the data base is from the US, even when the manufacturers and names of the products are the same, the recipes for each country are often slightly different. In the US they also calculate carbs differently. Inthe US fibre is included in the carb count, it has already been subtracted in Europe. If you use a US site then to get an comparative figure to the UK you have to subtract the fibre from the carbs.
if you want to record your daily diet, there is a site called Livestrong. com (my plate) This is used by people from all over the world. This means that there are far more products from the UK. (I've even found some from France!) The big problem with that site is that the data is occasionally not accurate (some has been put in my users). As long as you are aware of that, then I think it's more useful than calorie king.