"There is a 20% tax on turnover on EVERYTHING Apple sells in the UK. It's called VAT."
VAT is not a "20% tax on turnover".
It is a tax added to the price of the goods or services and is ultimately paid by the consumer. The company is merely collecting it from the end customer, and hands it over minus any VAT they themselves have paid on allowable goods or services they have purchased along the way.
"so they employ lots of staff in the UK, all subject to national insurance and so on."
They pay employers' national insurance contributions, yes - but that isn't a tax on the company, as such. However: "and so on"? You originally claimed they paid "tax and NI on local employees." The tax is deducted FROM the employees wages and salaries (along with the employees' own NI contributions). Apple isn't paying anything here (other than the ers' contribution as I said) - just like with the VAT, they are merely collecting it, this time from the employees, and then handing it over to the tax man.