As I understand it, an EHIC Card entitles the bearer to recieve treatment at the same cost as a local resident within an EU country. In Wales, residents are entitled to prescriptions without paying a prescription charge, but in England most residents pay an £8.80 prescription charge. What's to stop a resident of England travelling to Wales (albeit without the expressed intention of recieving treatment which would invalidate the card) and claiming free prescriptions using their EHIC card?
EHIC cards state the country which issued them, so the NHS staff in Wales would be able to see that the EHIC card presented was not from a different EU country and therefore not relevant.
On the more general question about avoiding prescription charges by travelling from England to Wales, the NHS cross-border services guidance states that to receive free prescriptions you need a GP who is based in Wales and a prescription dispensed by a pharmacist who is based in Wales.
Inconvenience of a travel usually costs more then 8.80 pounds.
Additionally you are wrong about:
the bearer to recieve treatment at the same cost as a local resident within an EU country.
On NSH page Cover your healthcare abroad:
If you're going abroad on holiday or a business trip, you should apply for a free European Health Insurance Card (EHIC), which may give you access to reduced-cost medical treatment.
So, if you go to your doctor and you get your prescription (so at your home), it will seems very strange that you will go on a trip forgetting to get the medicines (but not forgetting the prescription).
And abroad you could get some extra costs, not for the prescription, but to validate the prescription (e.g. for the chemist to call abroad; this case could be suspect, so the validation could be required).
Note: your question is not really about abroad (England and Wales), so the above is not relevant, also the EHIC rules are not required. In any case if you have a 8.80 surcharge in wales, you are still paying like people in the same country (UK).