Unsure of places to report this:
http://www.met.police.uk/computercrime/index.htm
http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=report+child+porn
This is a good start, and also send copies to the Metropolitan Police (Scotland Yard) in the UK internet crimes division as they act very fast. Also once sent they will take it on from there and communicate with interpol cross countries.
When sending the email include headers of the email otherwise it does not help them.