Your configurations of your tax classes is faulty. Check to see that ALL YOUR PRODUCTS have the correct (applicable) tax class being applied to them.
In the case of the UK, products can be VAT inclusive, or VAT exclusive (such as fresh food, books, kids clothing - where vat is zero rated).
So if you sell a range of items that include products WITH vat, and products WITHOUT vat, you need to make sure that your tax classes are set up properly, and when set up APPLIED properly to individual products.
Firstly, make sure that you only have TWO tax classes (if you are selling exclusively in the UK). These can be (for example):
VAT
VAT EXEMPT
As they are co-applicable, they must have the SAME priority number.
In your Tax Rates settings, you would now apply:
15% to VAT
0% to VAT EXEMPT
Then, when you configure your product data, you will apply the tax class VAT to product that must have 15% tax, and VAT EXEMPT to products that are zero rated.
The priority settings are important.
Also, get rid of any tax classes and rates that do not apply to the regions and zones you sell in.
The BOOK, by Go-Koon-Hoek, has a very good explanation of how to set up tax on ZC.