London, like the rest of the UK, has a temperate maritime climate with all four seasons, but no extremes of temperature. Here's a general overview:
Spring (March - May): Spring can be quite variable, with March often seeing some of the year's more unpredictable weather. By April and May, temperatures are rising, generally reaching around 12-15°C (54-59°F), although evenings can still be quite chilly. You'll start to see more sun, and the city's parks and gardens come alive with spring blooms.
Summer (June - August): Summers in London are generally warm with temperatures usually in the range of 20-25°C (68-77°F), although it can occasionally go over 30°C (86°F) during a heatwave. July and August are the warmest months. London can experience rain at any time of the year, but summer is generally the driest season.
Autumn (September - November): Autumn can be mild and pleasant, with temperatures usually in the range of 11-19°C (52-66°F), but it can also be one of the wettest seasons. The city is beautiful in autumn as the leaves change color.
Winter (December - February): Winters are generally cold and damp with temperatures usually hovering around 2-8°C (36-46°F), and it can occasionally drop below freezing. Snow is relatively rare but does happen occasionally. December tends to be the wettest month.
London weather can be quite changeable, so it's always a good idea to check the forecast before you go and to pack a light rain jacket or umbrella, just in case. The city is known for its occasional rain showers, but it's also known for the weather changing quite quickly.