For water I like a sawyer filter and 2 glacieu “smart water” bottles. They are extremely light, the threads fit the sawyer filter, and the spout/cap can be used to back flush the filter.

Freeze dried food and nuts/grains are popular, but not exactly tasty or healthy. If you are able to fish, that’s about the easiest and best tasting option. For packing in food, I would bring 1kg butter (best fat source, and you can fry fish with), jerky, and pemmican (carnivore bars, for example). Eating carnivore does use more water than a carb diet, so I wouldn’t in places with scarce water, but I’m sure the highlands have springs all over the place.

Freeze dried coffee is also the way to go, because it’s fast. Don’t get the cheap stuff. Black Rifle or Starbucks are the best of the instants, and they are more expensive, but it tastes like coffee. The Folgers, Nescafé and other cheap stuff are terrible.

Freeze dried meals are pretty much the best bet if you want to most amount of calories for the least amount of weight but that doesn't mean you have to have all 3 meals that way. Main thing to know is the amount of calories you need to survive, on a normal day at home and what you use when hiking with a pack are 3 very very different numbers!

My go too for breakfast is oatmeal, starting the day with something warm is nice when you are cold and packing up a tent, I love coffee but usually bring tea, last thing you want is to shit yourself frantically trying to dig a whole.

Lunch/snacks, pretty much anything you can eat on the go, trail mix, bars, jerky. Soup can be nice but getting your stove out and things cleaned up afterwords can eat alot of time up.

Dinner, this is where I may have a big Freeze dried meal or a bunch of beans and rice, its a good time to pack in the calories you lost form the day.

As for water I use a combination of a camelbak and bottles. The camel back is nice for sipping as you hike and the bottles are better when your are prepping meals and getting a good gulp in.
For filtering water I really like MSRs bags, it's not the lightest but is super easy and you can bring extra water back to camp at night.
Alot of other options out there, just don't be the idiot kneeling by a puddle with a life straw.

And lastly you have your first meal but when you get out. This is going to be one of the most satisfying meals of your life. Once did a week long trip with my wife and we packed a little light on food. On the drive home we stopped at the first pub had the biggest burger they had with a large side of onion rings, a bit later had footlong deli sandwich from the next town over and lastly when we got home we ordered in some sushi that would have fed a family of 6. Slept for about 14 hours after that.

Specifically Scotland they probably plan on using water from some creek. And the amount of calories in a kilogram of dried lentils/rice/beans is astonishing.

Or maybe they plan a route that intersects with a car road or settlement every second day.

Anyway, for places other than Scotland, let me tell you about these bad boys:

https://upload.wikimedia.org/wikipedia/commons/1/16/Bolderkar.jpg