This page has five screenshots taken from the BT Wholesale Performance tester that Max or Max Premium ADSL users have access to www.speedtester.bt.com. This tester can only be accessed using your normal ADSL ISP username, in which case it will test your connection and speed over the Internet. For those on an fixed speed product e.g. Home 2000 you can use the speedtest@speedtest_domain login (details here), which bypasses your service provider.
NOTE: The Performance tester site is very busy so several attempts may be needed to run the test.
For most people the test is useful to find out what your IP Profile setting is (sometimes called BRAS Data Rate), this figure is the maximum dataspeed you can download from the Internet on a MAX ADSL connection.
If you have any questions about the speedtester then a good place to get help is bbs.adslguide.org.uk.