While the temperatures are generally comfortable, we do sometimes have cold fronts or heat waves that take us by surprise and the polar front jet stream can make British weather especially difficult to predict.
You might also be surprised to know that contrary to what you see in some films, it doesn't snow very often in most parts of England. Every year we can expect some snow which may affect transport and allow you to have a snowball fight or build a snowman but as the temperatures are normally above 0°C (32°F) the snow doesn't normally last very long.
One thing that we do have more of than some other countries is hail. This is when little balls of ice fall from the sky, sometimes accompanied by a thunderstorm. Hail storms come and go very quickly and are normally a surprise to everyone.