Have you looked at data usage on the Android phones (under settings)?
My wife's phone suddenly upped it's data usage due to a Google "improvement" running continuously i the background>
Should you not have received an email from BT warning you you 'may' go over your limit?
Same here ! I had an e-mail on 28th Feb saying I'd used 36GB of my 40GB limit, & was charged this month because I'd exceeded it. That means I used 4GB+ over the space of a few hours, which I didn't. I have asked BT to provide evidence of the usage that supports why they have charged me, which they apparently can't do. So, by my thinking, if they cannot provide the evidence, how can they charge? Just about to raise an invoice to them for the charge + my time + calls, etc, myself !
We have an allowance of 20 GB per month. Last month we used the laptop in the same way except for upgrading to windows 10, which shot up our usage to over our allowance and we had to pay an excess. Our previous average allowance was 8.7 GB per month. This month, we have been very careful in our usage, but we are using far more than before getting windows 10. I am checking the usage on a daily basis. Is it possible that having windows 10 is responsible?
Windows 10 uses a Peer To Peer method to distribute updates, and will use your upload and download bandwidth. As BT add the two together, you are going to use a lot more now.
Here is how to disable it.
I would advise that you upgrade to unlimited usage if you are going to use Windows 10 a lot.