Traveling to london... what clothes should I bring?

I'm 15 and going to London for a week in the beginning of July and I don't know what clothes to bring. I know it's kind of chilly in London so shorts and tank tops like I wear back home are not an option. So I was guessing just some jeans and tops, maybe plain v-necks with a scarf? Does that sound good, or is it too casual? I have no idea what teens wear in London. I'd definitely bring some cardigans, and a skirt incase we go out. What about shoes? Are flip flops fine? And what do most teens wear? Are they high fashion or just pretty casual?