This is another back door method to cut what you get, and most of the ISPs use this. The cut off used to be 120 MB now its down to 100. You may also find it restricts BT vision It did mine.
I was on option 3 with my phone line it last bill without phone charges was £170 for a quarter. I Had already warned BT I would leave if they tried this again. Especially as the lines could only provide broadband at just over 3 MGS
Luckily I was out of the Min contract period. When this happened again in Jan I terminated and went to Virginmedia.Cable 10 meg, unlimited phone calls and a basic tv package. The BB speed test shows a full 10mg, no signs of a so called fair use policy, and under £40 a month. So savings of £50 a quarter