Straight-line distance to exchange (postcode based):
http://www.dslzoneuk.net/distance.php
Another checker, gives straight-line distance plus exchange info:
http://www.dslzoneuk.net/checker.php
More detailed exchange info.:
http://www.samknows.com/broadband/search.php
Another availability checker:
http://www.samknows.com/broadband/checker2.php
Loads & loads of info regarding speed, noise, & attenuation:
http://www.skyuser.co.uk/forum/speed...explained.html
Includes a table giving the approximate relation between attenuation, line length (actual, not straight-line), & possible maximum speed (subject to noise margin):
http://www.skyuser.co.uk/forum/speed...explained.html