Nope. Even when you are in receipt of state pension you have to pay.
I am very surprised you get free prescriptions as asthma is not on the list of exemptions.
Medical exemption certificates are issued on application to people who have:
A permanent fistula (for example caecostomy, colostomy, laryngostomy or ileostomy) requiring continuous surgical dressing or requiring an appliance
A form of hypoadrenalism (for example Addison's disease) for which specific substitution therapy is essential
Diabetes insipidus or other forms of hypopituitarism
Diabetes mellitus, except where treatment is by diet alone
Hypoparathyroidism
Myasthenia gravis
Myxoedema (that is, hypothyroidism requiring thyroid hormone replacement)
Epilepsy requiring continuous anticonvulsive therapy
A continuing physical disability which means the person cannot go out without the help of another person. Temporary disabilities do not count even if they last for several months