Yes! I've spent many a summer in England. My mom is English, so all of my maternal family is there, and my brother, my only sibling, has lived there for nearly 20 years now.
Over the years I've experienced all kinds of weather there. I've spent most of my time in the south of England (London and Surrey) and the weather can vary quite a bit. Some years I've gone and it's been cloudy all summer, and other times there has been a heatwave. All the advice about bringing layers is smart. I'd bring shoes that cover your feet as well as sandals. I don't know about rain boots. I bring them, but my cousins live on a farm, so I need them there. I've never worn them in London city in the summer.
It's probably not going to be particularly cold, but it might be cool. Spring jackets/sweaters/tank tops/tee shirts will do ya, I think. Hope you have fun! I love London.