I've had this email too. I'm on the basic BT broadband which means I get 10GB of usage per month. If you're concerned that the email is a scam you can go on BT.com direct and check you're usage.
I found I'd used 7GB of my allowance, with a week to go to the end of the month. I've only gone over my limit once and BT gave me a free pass, with a warning that they'd charge £5 if it happened again and if it continued they'd have to put me on a different deal.
At the time, I happened to be home more and had been using my computer during the day, as well as in the evenings.
So I think it's certainly worth double-checking on BT, it may be that since you've had an ipad which is more portable, that perhaps your usage may have increased from what it was previously??