The RHS does a veg calendar which shows when to sow and harvest veg:
http://www.rhs.org.uk/RHSWebsite/files/fb/fb14af03-f854-4cd6-8570-95b49c9e3451.pdf
Here’s another source of info:
http://www.thompson-morgan.com/sow-and-grow.html
London has a longer growing season than much of the UK so bear that in mind.