At this stage it is only an investigatory meeting – there is no assumption of guilt and it is just an opportunity for them to find out what is happening and ask you questions and at the same time for you to talk with them about their concerns. This is not a formal disciplinary so you are not expected to formally defend yourself. As such there is not much preparation that is needed or that can be done. You only know that the reasons for asking you to attend are that they believe you are not proactive enough so you just have to think about examples to shoe that it not the case.
In terms of dismissal, if you have been continuously employed at your place of work for less than 2 years then your employment rights will unfortunately be somewhat limited. Most importantly, you will not be protected against unfair dismissal. This means that your employer can dismiss you for more or less any reason, and without following a fair procedure, as long as their decision is not based on discriminatory grounds (i.e. because of gender, race, religion, age, a disability, sexual orientation, etc.) or because you were trying to assert any of your statutory rights (e.g. requesting maternity/paternity leave, etc.).
If the dismissal had nothing to do with any of the above exceptions then you would not be able to challenge it and your only protection would be if you were not paid your contractual notice period, because unless you were dismissed for gross misconduct, you would be entitled to receive your contractual notice period. If you did not have a written contract in place you would be entitled to the minimum statutory notice period of 1 week. Your employer would either have to allow you to work that notice period and pay you as normal, or they will have to pay you in lieu of notice.
So it is possible that they could dismiss you now or at any time within the next 6 months and you would not be able to challenge it. Of course it does not mean that they will but the option is there if they need it.
I hope this has answered your query. I would be grateful if you could please take a second to leave a positive rating (3, 4 or 5 stars) as that is an important part of our process and recognises the time I have spent assisting you. If you need me to clarify anything before you go - please get back to me on here and I will assist further as best as I can. Thank you