Tim: The impression I had was not that British law banned viewing child pornography under any circumstances, but that it had such a ban on downloading it. Do private citizens get to escape that rule too, if they have a "legitimate" reason for their actions?
Update: I did some digging. Downloading a photograph, by any definition of the term "downloading," is considered equivalent to making a photograph. Simple possession of such photos is also illegal, and has been since 1988.
There is an exception for "legitimate" or accidental possession of child pornography, but the courts are extremely skeptical and almost never accept this defense.