What I would like to know is whether there is a way to find out what our average internet usage is
I don't know about your plan but we also have internet with Plus through Cyfrowy Polsat and we have access to a website that tells us the details of our ongoing contract including how much of the limit we have used up - if you plan on downloading bigger things it is adviseable to get a night plan (works from 1 am to 8 am) and has quite a big download limit (in our case it's 200 GB per month) and it costs peanuts