There are things, like fruit and vegetables, where portion control isn't an issue. There are other things, though, on the approved Core list such as corn or baked potatoes or mashed potatoes, or beans and rice, or whole wheat spaghetti ... things that you could definitely overeat on if they happened to be a problem trigger food. Those are the types of foods where I would watch the portions and gauge my comfort level.
You've done a great job so far, too! I've never heard of slimming world but am going to check it out now.