I doubt they would cut you off mid-stream if you went slightly over anyway.
In the early days of broadband it was more common to have a data cap and if you went over one month is was usually ignored, make a habit of it and you got a gentle reminder letter. I would stick with what you have, if there's a way to monitor use and it looked worrying you could reduce the youtube resolution to 720p or 480p.
They would have to sit closer ;)