adsl and cable just work in different ways.
ADSL goes through your phone line down to the local exchange, Cable goes into a shared wire to the box in your street.
Two seconds of Google:
http://www.google.co.uk/search?hl=en&q=difference...ntryUK%7CcountryGB
http://www.seg.co.uk/products/adslover.htm
http://www.dslzoneuk.net/adslvcable.php