Speed limits only apply to motorised vehicles, there being quite a few practical reasons:
Cycles are not required to have speedometers, therefore it cannot be assumed that a cyclist will know how fast they are going;
Where cycles DO have speedometers they are not subject to an annual calibration test (in the MOT) and so their accuracy cannot be assured;
Cycles carry less destructive energy into a collision (though obviously being hit by a bike at 30 could have serious consequences);
Cycles, being smaller and more manoeuvrable have more chance if avoiding a collision.
Of course if a cyclist was speed gunned over the limit then it COULD be classified as dangerous cyclist (please not 'furious and wanton', that's ancient and doesn't apply anymore as far as I'm aware), but that requires a subjective view of the officer viewing the speeding as to whether the speed itself constituted dangerous cycling (taking into account all sorts of factors from the surrounding environment).
Anyway, short answer, cyclists cannot break the law on speeding because the law on speeding doesn't apply to cyclists (have had confirmed in a great response from the police a number of years back when I was writing an article on it, together with the weird mistaken belief that cycling offences can get people points on their driving licence).