It varies. When you go to maccy ds have you noticed they ask you if you're eating in or out?
The reason they do this is because the vat rules differ between the two.
Gregs offer hot food, which is eaten off premises. So IMO this should not fall liable to VAT and I can see this being taken further by the company. If they loose, this would mean a lot of companies are vulnerable to being taxed on many other sales which they currently are not.
There are millions of rules surrounding VAT which I will assume many on here won't understand as they're not clear in many cases.
My understanding of this particular rule is that gregs shouldn't.
IF the food was cold, and you heated it yourself it would not be liable to vat. It's the fact it is hot where the confusion, and the grey area arises. It is no longer a food and becomes more of a luxury.