I use this one: http://www.dslchecker.bt.com/adsl/adslchecker.welcome
I have more than just my own information to hand though... Before OpenReach embargoed the data, I used this map
to show all the local post-codes, and the identity of the cabinet that provided service for that postcode.
I have that info jotted on a printout from Google Maps.
Then I put the various post-codes that share my cabinet into the checker, and could see how the speed predictions varied for both ADSL2+ and FTTC. That let me find out just where my cabinet was located (and no, it isn't the closest one to me), and then work out the routing for phones lines springing out of that cabinet.
Finally, as BT have started offering FTTC service from my exchange, I can use the same knowledge (and a few local phone numbers) to identify which cabinets have already been turned on.