Food
Some items have zero or no tax (some food, children’s clothes), some very limited items are 5% but pretty much anything else is 20%.
However, if, for example, you go to McDonald’s for food and drink, there are some complicated rules relating to tax for hot and cold food and drink. Put simply, if you eat in, you pay VAT. Take away, and some items, especially cold ones, are 0%. That is why many places show eat in and take away prices on the menu. Some take away items are cheaper.
If you say you are taking away, you must take away.
No matter, the price stated is the price you pay. All the tax stuff is done for you.
Export
As you may expect, London has visitors from many lands. People come and buy stuff and go, taking their stuff with them ie exporting it.
In the old days, we used to go round hifi and electrical shops, especially Tottenham Court Road, and see items with two prices, the normal price including VAT and a lower export price. Presumably the VAT was removed or refunded later as the item was leaving the country.
We seemed to remember tailors, and probably other shops, did the same.
Duty and tax free shops at airports
Let’s not go there. How many bars of Toblerone do you need?