Helen, I know nothing about how to cope with England's socialized medicine scheme, nor what are the legal liabilities and responsibilities.
My first reaction is unhappiness at the news that you are having some troubles. I know that the physical problems are compounded by anxieties, and unfortunately to are being exaggerated by an incompetent and unfeeling bureaucracy.
In the US, should such a situation occur (and our medical practice differs from yours), a woman in your condition would get her doctor to declare her in an emergency condition, facing the threat of, I don't know, toxcemia or spontaneous abortion or eclampsia or something else requiring immediate attention in the hospital. then the woman would be admitted, and given whatever tests the doctor would order.
It may be that your doctor could light a fire under those people with some kind of emergency declaration. if you get the scans, and whatever treatment you need, and if it later turns out not to have been such a critical situation -- what can they do? And you will have gotten treatment, if you need it, and certainly you would gain peace of mind -- important for you and your incipient baby/ies.
Sorry for not much help, but I do want to express sympathy, concern, and hope for your situation.