[BB11UK]

Sorry, but this is just a sorry excuse used by people whose goal to be a housemate isn't going to come true.

Yes, many HMs have been test-HMs the year before, and yes many HMs have had agents before entering the house, but you're not taking into consideration that the test-HMs went through the audition process just like everyone else, they just didn't make it into the house (the final people either go into the test house, become a housemate or are a stand-in).
The test-house is filmed just like BB, but isn't broadcast. They're in their for 2-3weeks and it's the same as the normal BB, so arguements, romances, friendships, etc. all happen just like BB. As they have got to the final stage of the previous year's auditions but not made it onto the show, and have now been seen by producers in the test house - it allows the producers to go "actually, they'd have been a brilliant housemate" and so they ask the participant to try again next year.

About the agent - yes, many of them have agents before entering the house, but a lot of them are advised to get one just before they enter so that someone can handle the publicity while they're none-the-wiser for 3months in the house. People such as Noirin got an agent just a few weeks before she went in, not a few weeks before she auditioned.