A PhD won't qualify you to practise as a medical doctor: currently the only way to do that in the United Kingdom is to gain a GMC accredited Bachelor of Medicine, Bachelor of Surgery (MBBS, MBChB, BMBS, and a thousand other abbreviations for the same thing), or to do an equivalent degree abroad, but that route depends very much on where you do the degree and I'm not sure about the intricacies of that way!
A PhD is a research doctorate; it qualifies you as a researcher. If you want to go on to work in academic research in the future, a PhD is a great choice, or you can even get jobs in industry afterwards, however, a PhD is NOT a route to becoming a practising medical doctor.
PhDs, in addition, are usually very highly specialised. A girl in my year has a PhD in which she spent the time researching something to do with cathepsin K in a certain type of lymphocyte - I can't remember what it was she did exactly. You don't study for a "PhD in physics" or a "PhD in medicine" quite so broadly - it's much more specialised than that: you'd be applying to PhD programmes focused around immunology right off the bat! If you want to become the type of immunologist who's a scientist and works in a lab doing research, then you have to go this PhD route!
If you want to become a medical doctor after doing another degree, the route you have to take is
Graduate-Entry Medicine - this course is 4 years long. Alternatively you can apply for the standard medicine course as a graduate, which is 5-6 years long. You learn the same stuff in GEM; the course is just compressed by a year.
I wouldn't count yourself out quite so quickly from doing medicine straight out of school either.
Here are the minimum GCSE requirements for all the different medical schools in the UK - are you sure you don't meet them? What are your GCSEs?
If you want to become a
clinical immunologist, i.e. the type who treats patients, then you have to go to medical school to obtain the Bachelor of Medicine, Bachelor of Surgery degrees that I mentioned above (4 years if GEM, 5-6 years if the standard route). Then spend 2 years as a junior doctor - rotating around various medical and surgical jobs - before going onto specialty training, which for
Immunology seems to involve doing core medical training before going onto specialise in clinical immunology.