There are many ways of doing this, all of them nerve racking! My wife posted her documents to me in the UK via Thailand Post. I then put together the application and sent to Sheffield special delivery with return postage included via special delivery. You are supposed to provide this. It clearly says this on the website.
I received the documents back within two weeks, in loose form (no folders). Inside was the standard letter saying that they had scanned the documents etc We had no other communication until the visa was issued.
My wife was issued her visa just inside the magic 12 weeks. So be patient and plan accordingly. You can book flights once it has been issued.
It may be worth contacting Sheffield about the return postage. You could send it to them at a later date. You don't want them sat on the shelf gathering dust, or heaven forbid lost!