Is it prohibited to cycle on a road if a cycle track is available?  Yes, and there are prosecutions for it.  In a test case, we might argue that a cycle track is NOT ‘a portion of a road’.

Road Traffic (Traffic Control) Regulations – Regulation 51, Additional rules for bicycles, tricycles and rickshaws

(5) Where a portion of a road is set aside for bicycles or tricycles no person shall ride a bicycle or a tricycle on any other portion of the road.

A TV API (c2009) told cyclists they must use a cycle path [if there is one].  Tone was negative: ‘you cyclists have to be controlled’.

Compare UK, where the Highway Code says that “Use of cycle lanes is not compulsory and will depend on your experience and skills, but they can make your journey safer.”