I think it depends on what your attitude is. I was friendly and super excited about being there, which ultimately made my look as a happy and positive person. I know from people who traveled to South America that you guys are positive folks in general.
I was trying to chat with natives form day 1 (I knew maybe 15 words ! ;) and most of them were ok with it. They seen my efford and were happy to help, even just by listening my babble. That's cool, ha ? Yes !
I wouldn't worry about that. There are people from all over the world. At least in London and other big cities.
Which city would you like to go to ? How good is your English ?