Cell formatting not working for table header

Hi,

I'd like to put a value in a cell (say 20%), and have Excel show a piece of text, such as VAT (20%). So I go to custom cell formatting, and put in "VAT "(0%). This works for every cell in my excel file, even for a cell that is within a table, but for some reason it won't work for the header of that table. In the formula bar it gives me VAT (20%), instead of 20%, so I can't do any calculations with it. What might be the reason?

Thanks,
EG