Section 18 would be the right charge. It's not attempted murder because if he had been seeking the death of the cyclist he could have followed it up and acheived it.
I can see how a defence lawyer could argue it down to Section 20 with a pathetic but hard to argue 'moment of madness' defence, saying 'he didn't even think, there was no true intent to injure this severely, he just lashed out, he's devastated at the consequences' etc etc.
Regardless, 17 months is lenient considering how long he will actually serve... but our justice system has developed to the point of under sentencing for everything IMHO.