London's weather is cold and damp in winter and bright and warm in the summer. The average conditions are as below:
London in Spring (Mar - May)
Quite unpredictable with a good mixture of sun, rain and cloud. Temperatures average between 11°C and 15°C. Take a jacket when you go out.
London in Summer (Jun - Aug)
Generally sunny, with the occasional rainstorm. Temperatures average around 20°C, but have been known to go as high as 37.9°C.
London in Autumn (Sep - Nov)
Pretty much the same as spring. However it's not unknown for summer to run well into September or for November to be uncharacteristically cold. So be prepared.
London in Winter (Dec - Feb)
Temperatures hover around 6°C and it's normally mild but damp. It rarely freezes or snows in London, but you'd be wise to take a hat and gloves when going out.