What's the weather like in London?

Hi guys I'm an Italian girl and at first I have to say sorry to everybody for any mistakes I'm going to make! Well on Sunday the 2nd I'm flying to London and I'd like to know what the weather is like! I'm from Rome, here is very hot and sunny so I'm not sure about what kind of clothes I have to wear there. I've already seen the weather forecast on the web but I don't really trust it, I trust you more! Anyway I've seen it's going to be rainy and clouthy and the temperature is going to be between 10 and 20 degrees, is it true? Please let me know and remember that for Italian people 10-15 degrees is quite cold! Thanks a lot :)