Well, we also have a freedom of information legislation, thats all. If your supervisor puts things in his reference that are wrong you can ask him/her to correct it (and go to court if he doesn't). Most labheads in the field know each other anyway, so if your supervisor is going to knock you down infront of others you can't do much anyway.
For example, one labhead here introduced two PhD students as "big tits and small tits" to other labheads (and possible future employers) during a big conference with all the important people there. 'Course he looked like the real idiot then ...