What you say is not quite right, Steve.
If you plead your innocence and the prosecution are unable to produce sufficient evidence to prove your guilt beyond reasonable doubt (quite a heavy burden), then you will be acquitted. In short, the good old British justice in which you seem to have no confidence, is still likely to prevail.
The fixed penalty system is designed to allow those that are guilty to avoid court action (and all the extra cost that entails, some of which will be borne by the defendant if convicted). It allows a considerably reduced penalty to be accepted than would be applied if a court hearing with guilty plea were held.
There is a considerable difference between pleading innocence and actually being innocent. There is therefore no discount for pleading innocence, only a discount for accepting guilt. There is, however, 100% discount for being innocent.
I�m only a simple soul and I don�t really know how else it could operate, but maybe you know differently.