McCall's General Pattern Sizing Question Sewing Machines Parts and Sales in Los Angeles

Hi, sorry if this is really basic, but I have a question about which pattern to buy. I’m a UK18, and the McCall’s pattern I’m looking at goes to size 14, and then there’s a second one that goes to size 22. I was going to get the larger sizes to be safe, but I just need to know if the sizes are in the UK or US before I actually cut anything.

Also, on a skirt like this, what makes it go out and then back in? Is it the sort of fabric, the pleats, or the shape of the pattern?

TIA 🙂