After the shock of seeing how much data my laptop was sending I started to look for ways to monitor the amount of traffic.
The first useful program that I found is called Bandwidth+, which is available through the Mac App Store. This puts a counter in your status bar that shows how much traffic you have used with options for only showing upload or download, by network and so on. This was a pretty good start but it is only passive, you still have to manually stop applications if you are at risk of blowing your allowance.
Not long after I returned home I stumbled across another tool, Tripmode, that is even more useful. Tripmode monitors which network you are attached to and, via a drop-down menu in the status bar, you can toggle on/off which applications may access. Quick & easy.