No, they are specialist nurses employed by PCUK - they provide telephone advice, can put diagnosed men in touch with other men that have already been in the same situation, and can send out hard copies of the toolkit to people who can't download it from the website for some reason.
Macmillan nurses work in different ways depending on where you live. In some areas, they pick up cases where someone becomes terminally ill, providing nursing care, pain relief, financial advice on the benefits that can be claimed, etc. In some areas, they work out of the local hospice. In other areas, they are employed in the hospital to provide care and advice alongside the oncology staff.
The nurses that you have heard of but never met are, I think, the clinical nurse specialists. As you have picked up, some members here have amazing CNS who basically run the outpatient service - telling men their diagnosis, explaining the treatment options that are on the table, arranging ongoing PSA tests (and phoning with the results when they get them), keeping in touch and dealing with questions, meds and any post-treatment problems. Sadly we don't all have the same provision - where I live, my husband, dad, father-in-law and mother-in-law were all under the same CNS but we have never met her and she has never answered the phone or returned a message. I assume she is very nice but very busy :-/