The £14.99 offer was originally only made to Tiscali dialup subscribers and was for 1MB with a 2GB limit.
Shortly afterwards the limit was scrapped for both the 512 and 1MB service at this price.
You may be served by an exchange which is just outside the optimal distance from your property but still able to get at least 512kbps following BT's upgrading measures.
Be somewhat thankful. It's still up to 10x faster than dialup.
I have the 2MB service (2GB cap) at £15.99 and find it excellent; it was updated foc from 1MB but, when I originally switched from Tiscali's DayTime Plus, I was getting speeds of up to 2.4MB for nearly six weeks although signed up for 1MB...:-)