you're ignoring the 20% VAT on those 3bn sales, which provided 600m tax revenue. Why is it so important that they paid 7m in corporate tax instead of any other amount? Their business apparently has high cost of sales (like stores, personnel etc.), where by the way also taxes occur, e.g. for wages...
So I suggest to think twice if you want to paint the picture that Starbucks does not contribute it's fair share to taxes in the UK.