The BT policy on download allowances changed several months ago. Most BT packages now have no download limit. Now only the cheapest package has a limit - 10Gb.
A warning email was sent the first month that the allowance was exceeded and then the customer charged if they exceeded their allowance the following month. Now there is no period of grace. A customer is charged £5.00 for every 5Gb (or part thereof) that the 10Gb download limit is exceeded.
See here
Learn about your Usage Allowance | MyBT Broadband