Suppression of Protest, a Catholic value...

http://www.cnn.com/2010/WORLD/europe/09/18/uk.pope.threat/index.html?hpt=T2

Of course they were released. I highly doubt they had any real evidence against them. Christians are the ones who react to their opponents with violence, if anything atheists put up with too much shit without doing anything about it. They just wanted as much as possible to keep those who would protest the Pope off the street. They obviously can't arrest Hitchens or Dawkins so they targeted the less visible enemies of Ratzinger.

They were arrested under the Terrorism Act 2000, which allows police to hold them without charge for 28 days.

This is a really scary sentence. I imagine if Ratzinger had his way they'd have arrested every atheist in England and held them for 28 days.