What other people have said is very true, if you're set on South America you're going to have to be pretty good at Spanish. As soon as you get away from the cities, you're relying on your ability to converse in Spainish and if you can't then well, you're screwed basically.
If you're set on going to South America without a word of Spanish, then you don't really have the option of doing it all independently by just sending a few emails or turning up and seeing what happens. Thats why all of the companies exisit- they know Brits want to go to far flung corners of the world yet hardly anyone speaks a foreign language, so you pay them to organise stuff you can't do yourself due to your linguistic inability.
Even if you do go with them, it'd be a hell of a lot easier if you can speak Spanish. I was in Central America for several months a couple of years ago and I hate to even imagine being there without any knowledge of Spanish, as mine is pretty good.