I'd tell BBC "You and Yours" or Which? mag (Consumers Association).
That's an outrageous penalty for going over the allowance. I use up to 2000 MB a day on one of my (unlimited) Three contracts.
I thought Three was expensive (when I had a 15 GB monthly limit on a USB dongle, when they charged 10p per MB for any usage above the limit), but in your case it looks like 8 pounds a MB.
If you have an Android phone, I can recommend an App called "Plan Monitor" which tracks minutes, texts, data (mobile network) and data (wi-fi). It will disable the mobile data if you set the limits and warning/cutoff right.
I disable data on some of my phones (with PAYG SIMs) but have two mobiles on Three with unlimited data (13 quid and 15 quid - the second allows tethering for laptop, desktops and other items to share the mobile connection).