When I switched to BT mobile about a year ago, the cellular update to my iPhone *removed* the feature to turn on and off 4G for certain apps. This meant my YouTube app was stuck permanently without data, which was ideal, I know how much data video takes up and there's no point wasting it.
However, I recentley upgraded to an iPhone 6, and presumed the same settings were still applied, as I *still* don't have the option to turn on or off data for apps.
Imagine my horror, as when I wake up this morning abroad, that my wifi tripped off in the night, and YouTube decides to auto play videos. BT make no effort to switch off my data despite me going over the cap at about 4am this morning, then drains me up to my £10 limit within 20 minutes.
I even spent £6 on a 1gb UK add on to avoid going over my data limit before it resets tomorrow, so BT have let me waste £16 worth of data because they removed the feature to disable mobile data for some apps. I asked BT about this bug before and they said they would look into it, now it's cost me.
I don't think you can quite blame BT for your presumptions nor the fact you left your phone playing a YouTube video while you slept nor the fact you have autoplay switched on. Imagine if YouTube videos didn't work and autoplay kept stopping by itself - I guess that would be BT's fault also?