> For instance, is it illegal for police to hold images of child pornography as part of an investigation? Is it illegal for a child to have pornographic images of themselves?

> Still, it should be important to differentiate between possession, distribution, and production in all of these cases.

Luckily the relevant law does what you want and answers your questions. You'll want to read it with the sentencing guidelines too.

http://www.legislation.gov.uk/ukpga/2003/42/contents