All of the celebrity clones came from lookalike agencies in the UK, apart from Gareth Gates who played himself (he is the only celebrity clone to have any proper lines).
Jo O'Meara was suffering from back problems at the time resulting in her doing less physical activities (walking out of the prison while everyone else runs out, not appearing in the shower scenes, sitting on the sidelines while the group is rehearsing, etc.).
The concert scenes were taken from the S Club's Carnival Tour when Paul Cattermole was still in the band. You can clearly see seven people in the supposedly live show instead of the six that are in the movie. Also, hairstyles for Jo, Jon and Hannah are completely different than they are in the movie.
In the scene where Rachel is threatened by Alistair with a broken sword, she is first being grasped by him. In the next scene she is sitting back at the table with the rest of the group.
When Clone Rachel brings Bradley tea and biscuits and Tina is lying on a chair covering her eyes with a book. On the wide-shots the book is upside down; on Tina's close-ups it's the right way up.