And if the worst happens...

http://lcc.org.uk/articles/what-to-do-in-case-of-a-collision

The link takes you to London Cycling Campaign guidance on what to do if you are involved in a collision.  You may like to print it out and stuff it in your saddlebag.
Belonging to a national cycling organisation such as the Cyclists Touring Club (CTC) or British Cycling (BC) gives you access to expert advice and assistance in following up an incident.  The London Cycling Campaign is also a big enough organisation to have experts available to help members.