Hi,
I had the same doubt regarding the form, when I received the test invite. I eventually decided to take a photocopy of the form first and then fill out and sign the copy they sent me. I carried both for the interview (in case they ask me to fill up and sign in their presence). When the officer asked me for the form, I presented the filled up and signed one and she took it with no questions. So I am pretty sure they are ok with the forms being filled up and signed ahead. But to be on the safe side you could take a photocopy before filling it up and take that too with you. Hope this helps.
RSV