mowmow, your GP has behaved disgracefully and I am enraged on your behalf. You have been refused appropriate treatment AND in essence called a liar, (but that is impossible"). No, it is not impossible.
On the website of the Pharmaceutical Services Negotiating Committee:
There are instructions to prescribers as follows:
"If you have been unable to obtain the licensed product and wish to supply a special, to ensure correct reimbursement the prescription needs to be amended by the prescriber to specify the name of the specials manufacturer. The prescription should be fully endorsed and placed in the appropriate red separator for submission to NHS Prescription Services."
So you see, it can be done, and she would/should have known this all along.
There is a phone number for an update on the supply situation.
Take fedupsusie's advice go back and demand a replacement armed with the above, for no doctor has any right to deny you the medication you need in the form that you need. Your doctor has displayed the most amazing indifference to your needs which is tantamount to medical neglect.
You know, in such situations as yours, you can often get much more reliable information from your pharmacist.