Hi,
All PGCE interviews are a bit different, but from the three I went to, they seem to all be looking for potential rather than you to be the finished article.
For the teaching session/presentation, I'd say confidence and preparedness are key. You may be asked to reflect on what went well/badly in your individual interview. I don't think the actual content matters that much, but one of the biggest problems I saw at my interviews was pitching the content too high- if your target audience is, say KS3, think about what they may/may not already know and how to make the subject accessible to them.
The written exercise is mostly to check your written English as far as I could tell- so I guess clear communication and good spelling, punctuation and grammar are key. Readable handwriting is also good!
I only had a group exercise at one interview, but I think it's about showing good communication skills, as well as what you know- so drawing in those who aren't speaking as much, and letting others have their say while still putting your point across is good, I think.
The individual interviews were all really different, some were definitely more formal than others. You're likely to be asked to reflect on other parts of the day. You might be asked some subject knowledge questions. You might be asked some "tell me about a time when" type questions or about your resilience and time management skills. You're likely to be asked about your school experience and obviously your motivations for becoming a teacher (and why this age/subject).
I hope this helps a bit, and good luck!