When applying for my wife's UK resident visa in Buenos Aires we had to prove our relationship had been going on a while.....letters emails etc. It can be quite embarrassing showing evidence of your personal relationship. I felt it an invasion of our privacy but it had to be done. I also had to show bank statements as well as a letter from my employer showing my employment status.
We were also interviewed separately to see if our stories matched. Most disturbing for me was the fact they had asked for several hundred dollars before the process started and told me it wouldn't be returned if the visa application was refused.
I'm sure it isn't as intense as that for a visitors visa but be warned for the future ahead.