I am afraid that I don't subscribe to the unhelpful and arrogant attitude that one member seems to love repeating on here. It is absolutely unacceptable to fine people on a detail of paperwork, when an underlying condition means that medication is exempt by law.
I would ring and politely ask to be refunded and if threatened with court I would simple go for it. No court in the UK would uphold a fine regarding this. - it would be thrown out in seconds.
I'm not sure you do have a right by law to an exemption in England. The wording on all the NHS websites describes it as an entitlement, and very clearly states that you need to have
BOTH the condition and the exemption certificate.
On the basis that it is an entitlement and that it is stated pretty clearly, a court would probably uphold the decision on the basis that ignorance is no defence.
Whether I agree with this point of view is another matter, but as people get off charges due to technicalities, so does it work the other way.
The NHS is going after people who, according to its rules, are not entitled to free prescriptions because they have a condition that allows them to request an exemption certificate but have not done this.
Whilst it's not pretty, it's a probably one of the easiest and simplest cases to close down quickly from an investigatory viewpoint. I believe the term is "Bummer".