NHS will test for haemochromatosis (transferrin saturation and ferritin) - which is iron/ferritin as you know and nothing to do with B12. Haemochromatosis is usually genetic and the treatment is to give blood (or have a lot of blood tests). It is a serious condition. I've seen recommendations to avoid vit C (as it aids iron absorption) but not B12. You still need folate and B12 (and iron) to make new red blood cells.
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