If your cabinet really is 'just at the end of the road', then there sounds like somethings amiss. My first port of call would be to contact BT Business broadband, and have them get an engineer out, line I went to the other day had been poorly/incorrectly installed, giving the punter a sync rate of around 20 meg, and 40 when I'd finished.
p.s You
are running these speed tests when connected to the router via an ethernet cable, aren't you ?