snacks and food, what else can I do?

Meal deal. Virtually all British supermarket chains (and some non-supermarkets) offer a meal deal. This will typically consist of a pre-packaged sandwich, a snack (usually a packet of crisps or small chocolate bar), and a drink for a set price of around £4.

Some popular sandwiches you'll see are:

• cheese and ham (the cheese would be cheddar),
• cheese and onion (the cheese would usually be a mixture of grated cheddar and Red Leicester),
• bacon, lettuce, and tomato, and
• Coronation chicken (it's a very strange combination of cooked chicken, raisins, and mango in a mild creamy curry sauce. Look up recipes online. It's weird but it works.)

Probably also include a few extra sandwiches you think will be popular locally. Even if they seem very non-British varieties, that's fine, as most supermarkets will have a few sandwich options inspired by Asian food, Mexican food, Middle Eastern food, etc.

And a few non-sandwich main options like a tub of tomato pasta with a disposable fork.

For the snacks, any individual sized (about 25g) packets of Walkers crisps, Mars bars, Snickers, Dairy Milk, Bounty, etc. There will usually be a few healthy snack options too like a small pot of fruit salad, a whole apple, banana, or orange, etc.

For drinks, cans of Coke, Fanta, lemonade, Pepsi, etc. Small bottles of water, orange juice, apple juice, chocolate milk, whatever is popular locally.

I think if you can do something like that, you'd definitely be providing a very genuine British supermarket experience.

I cannot hang a Union Jack because my country won't allow it.

I don't think I've ever seen a supermarket with Union Jacks flying, so you've already got the authentic British experience covered there.