Hm. Have you had the bowel cancer self-screening test thingy? I think I've done two now (can't remember if it's every year or every other year). I don't know if you can request them, or get tested by your GP, but it might be worth a shot if it's something you suspect.

I was very marginally anaemic a couple of times when I was giving blood, so I put myself on iron tablets for a week before each donation and a week after. I found that the ones they gave me at the clinics gave me black diarrhoea (ick), but discovered that Ferrous Gluconate, which I ended up buying over the counter as it was cheaper than prescriptions (back in the days when I paid for prescriptions!), didn't, so I could take it without problems.