Asthma is not included on the list of exemptions which entitle people to free prescriptions, and I believe there are no plans to change the list of medical conditions exempt from prescription charge. 

However, there is also an extensive system of prescription charge exemptions in England. There are provisions in place for people on low incomes who can apply for free prescriptions through the NHS Low Income Scheme, or who get free prescriptions due to the receipt of certain benefits. 

Prescription charges generate a valuable income to the NHS budget of several hundred million pounds per year - money contributing to patient care that would have to be found elsewhere if these charges were abolished.