I'm going to be in England all summer and I'm totally stumped on what I'll need.

I know it's a bit far off but I feel like I should start shopping now and I just have no idea what to pack for the whole summer. Is it really always gloomy there? Do I need to bring warm wintery clothes just in case or should I be fine with lots of layers?

And should I bring mostly boots or flats?

Please help!