Ey, kinda late but whatever.
What I realised from last years exam, is that is prettu much guided you on what you need to write persuasively.
When I was given the persuasive piece exam, it came with the topic and a sample introduction. Most tutors just give you the topic but during the exam they gave us a sample introduction.
This means you can either go above and beyond and think up your own arguments, or leech of the sample introduction for ideas as the sample introduction is quite solid. It clearly states two arguments and a rebuttal for it, so I wouldn't worry if I were you.
I was pretty confident on my mathematics so I decided to play safe and not take risks (especially since I was only going for NHS) by just using the arguments "provided" by the sanple introduction. Worked out well, got like a high or above average (i forgot) and I got in... sooo yeah.
Overall, don't worry too much about arguments, just ensure you can make a somewhat decent argument on the spot even if it is stupid, as long as you can back it up in the explanatiom and evidence, you should be fine.