How big of a jerk will i look like...?

I'm a Yank and I have a Generalized American accent. How big of a twit will I look like if I attempt to use English slang in England?? And yes when I do use English slang it sounds as forced as the ironic previous sentences. How would it make you feel if I talked like this to you??? Please no scalding answers but I won't be offended easily :) Answer truthfully!!!