This page gives monthly averages throughout the year for London: http://www.bbc.co.uk/weather/world/city_guides/results.shtml If you look at the maximum and minimum temperatures for March (high 21, low -8) you will notice that you need the full range of clothing, from sunhat and t-shirt to fur-lined knickers and ski-jacket, because one of the wonderful things about the UK is that you can NEVER predict what the weather will be like!! On the whole, you can be fairly sure that it will be cool most of the time and that it will rain often.

Mild. About 10 degrees Celsius. http://weather.yahoo.com/england/greater-london/london-44418/?unit=c