From the Wikipedia page on London 
London has a temperate oceanic climate (Köppen: Cfb ), similar to all of southern Britain. Despite its reputation as being a rainy city, London receives less precipitation (601 mm (24 in) in a year), than Rome, Bordeaux, Toulouse, Naples and Sydney. Temperature extremes for all sites in the London area range from 38.1 °C (100.6 °F) at Kew during August 2003 down to −16.1 °C (3.0 °F) at Northolt during January 1962.
Summers are generally warm and sometimes hot. London's average July high is 24 °C (75.2 °F). On average London will see 31 days above 25 °C (77.0 °F) each year, and 4.2 days above 30.0 °C (86.0 °F) every year. During the 2003 European heat wave there were 14 consecutive days above 30 °C (86.0 °F) and 2 consecutive days where temperatures reached 38 °C (100.4 °F), leading to hundreds of heat related deaths.
Winters are generally cool and damp with little temperature variation. Snowfall occurs occasionally and can cause travel disruption when this happens. Snowfall is more common in Outer London. Spring and autumn are mixed seasons and can be pleasant. As a large city, London has a considerable urban heat island effect, making the centre of London at times 5 °C (9 °F) warmer than the suburbs and outskirts. The effect of this can be seen below when comparing London Heathrow, 15 miles west of London, with the London Weather Centre, in the city centre.