It says not to fill in pseudonames, but I have a name that I am known as, as an artist. This is the name I sign off everything i do, and this is the name people know me by. Am I allowed to use it, or do I have to use the name on my birth certificate?
A name that you use that is fictitous, particularly used in artistic work is practically the definition of pseudonym. If you were selected you would be entering into a contract with C4 so you'd have to use your real name. I'm sure you don't sign cheques with your pseudonym or expect to get a loan.
But saying all of that I don't see why if you were chosen they would ever refer to you as anything other that your pen name.
SO you shouldn't have to worry if you are signing everything Wolf Steel when your real name is Cuthbert Wetsapants.