Immigration officer questions if you go to interviews

I am from a Commonwealth country and I have applied for military service in the United Kingdom, and in the medical stage, I did it a couple of days before and now I'm just waiting for an interview call, dates that are not confirmed. I have already booked my non-refundable ticket and I am flying on May 29, so I have some email proofs that show that the visitor's purpose is genuine and I have my visitor visa from the base of my interviews, but I am little worried because I have all the documents and emails from my recruiting center, which shows that the purpose of the visitor is genuine, but little concerned about the dates of the interviews that I can ask … I already have booked my return ticket and Friend's address where I'm going to stay etc …
Has anyone done this experience before, if so, kindly suggest what documents will be required and what kind of questions will be asked of the immigration officer?
We await your comments and thanks in advance.
Regards ๐Ÿ™‚