Change circumstances

Hi david ,
Short answer: No
DWP can't insist on treatment or penalise you for not undergoing treatment. DWP administers benefits not treatments.
Although you have a duty to inform DWP of a change of circumstances, you don't need to report changes in medication or treatment unless an improvement in health has occurred. The only reason DWP say you shoukd report a worsening condition, is to allow you to achieve more benefit if you are more disabled. If you already get enhanced mobility and or Daily living then you won't get more benefit so worsening of condition is irrelevant.
Medicine or exercise is often prescribed to help manage conditions. Unless I have misunderstood your post , the use of the bike will just help keeping symptoms at bay or lower, fir longer. Presumably your walking ability or eg ability to cook or wash has not improved because you use the bike? If this is correct you do not need to mention this. Obviously if your disabilities are less meaning that you no longer can be awarded sufficient points to achieve the level of PIP that you currently get, then you MUST report this to DWP who will conduct a review.
The danger in volunteering the use of the bike that DWP will automatically assume because you use a bike therefore can walk and do all other PIP activities.
I hope this helps.
LL26