29yo english girl needs help understanding....

The simple rules are

if you eat less calories than you burn, you lose weight, that is simple physics and not debatable. Now it is hard to do that sometimes.

Less processed and whole foods fill you up more. Sugar and highly refined foods make you hungry 30 minutes after eating, that's why coke doesn't fill you up like an apple and yogurt which both have about the same cals.

you should shoot for 50/25/25 cals from carbs/fat/protein, or on that order and it depends on how active you are. If you shoot for 1500cals/day and don't add any cals for the exercise you do (unless you are doing something moderate to vigorous for over an hour) you will loss 1 or so lbs per week, or a stone every 2/3 months. This is a health rate.