For a court to convict a person of dangerous driving, the prosecution has to show that
(i) the way he drives falls far below what would be expected of a competent and careful driver ; and
(ii) it would be obvious to a competent and careful driver that driving in that way would be dangerous.
The Crown Prosecution Service advice to its staff (which is usually based upon the outcomes of test cases) is that 'speed, which is highly inappropriate for the prevailing road or traffic conditions' is an example of the type of driving which should result in a charge of dangerous driving.
If the CPS can show that conditions (i) and (ii), above, are satisfied (having regard to the statement about excessive speed), the court should convict you. However, if you can show that either of the two conditions was not met, the court must acquit you.
http://www.cps.gov.uk/legal/section9/chapter_b .html#14
Chris