In British English, that kind of use of 'will' tends to be for extra emphasis. For instance, in the sentence you have written, you put 'will' in italics; well, that's almost like the way it would be spoken - with the emphasis on the word 'will'. For example, if you said, "If you keep littering here, they will leave here", that is more of a statement of fact, although with an undertone of disapproval. Whereas if you said, "If you will keep littering here [emphasis on the 'will'], they will leave here", it's a kind of... how to describe... dismissive phrase (as though the speaker doesn't really expect the person to stop littering) almost like it could be accompanied by a shrug - imagine it ^^, yet at the same time a little bit more emphatic because it has a 'this is your last warning' feel to it - like "If you will keep littering here..." - *shrug* - "there's not a lot I can do about it so I'll let you go your own sweet way but be warned, they will leave..."