One more data point: I have a MEDIUM LEL 2017 (and 2013) jersey, which is just slightly tighter on me than the LARGE PBP 2015 jersey. Basically, you should order one size up from ForceGB, because the Italian sizes are fitted.
As for the gilet, I have a large one of those, too, which sits nicely over the top of a jumper and jacket when I'm standing in a force-nine setting off riders on any of our Cambridge events ... I don't like well-fitted gilets when riding, especially not in the summer.