There are many types of data which is considered "personal" (email, real name, ...).
What is not considered personal data? Is the username of a person personal data if it can be changed on different websites?
I'm no lawyer, but under the current UK Data Protection Act, personal data is broadly defined as:
"any information relating to an identified or identifiable natural person ('data subject'); an identifiable natural person is one who can be identified, directly or indirectly, in particular by reference to an identifier such as a name, an identification number, location data, an online identifier or to one or more factors specific to the physical, physiological, genetic, mental, economic, cultural or social identity of that natural person; "
You can see here:
So, if a username clearly identifies someone as a person, you might have a problem. But if it doesn't you will likely be safe. Better to be safe than sorry: find a legal consultant to verify.