I'll defend the suggestion to keep a copy of the app foe yourself. Keep it somewhere safe and it will help you next time FLR or ILR comes around. I kept a little cheat sheet with the basics - date we met, where we met, when we started dating, etc. I can't remember what it asked but they were silly things that didn't roll off our tongue. Having it written down made the application that much easier to fill out.
We had two in-person appointments and didn't make copies for either. We went with the theory that if we made copies, they'd have to be sure the copies were actual copies and not fudged. It saved us time at home, ink and paper, but did take a bit longer at the appointment. That was a few years ago, so they might think differently of copies now since there are more supporting docs. Others who have replied already have gone through the process more recently than me.
I would keep a list for yourself of originals you definitely want to get back. We sat in the cafe after getting our approval and sorted through the docs to be sure we'd been given everything important back.
We wore casual smart clothes. It's not an interview, just an application. Nothing to stress about dress-wise, but wearing nice clothes did make us feel a bit more comfortable in the situation.
Good luck!