What's the weather like in London in August?

The weather should be the best in July and August, with little rain. If it is after 10, there will be a lot of rain.

The temperature is basically a little over 20 degrees, and short sleeves can only be worn when the sun is shining at noon.

Very comfortable.

I suggest you buy a jacket after you go. It's windy in England when it rains, and umbrellas are decorations.