Of course the store doesn't like it - they still have to clean up after you and you are using their facilities, taking up a table a paying customer could be using.
VAT is paid on ALL food and drink served in the restaurant, unlike the take away food which may or may not be subject to VAT.
It is not a law, it is store policy to refuse customers to eat food bought elsewhere. The store is within the law to enforce such a policy and if you refuse to leave when asked you are then trespassing.