I have low ferritin. I was prescribed a large amount per day, but I choose to take slow release tablets twice a day (45 mg). They don’t upset my stomach, unless I’m already having hunger pangs when I take a pill. Other wise, I take one in the morning an hour before I eat, and two hours after my final meal of the day. It’s less than what the Dr prescribed but the side effects of regular iron pills affected me poorly. Make sure you eat a lot of fiber during the day tho. Not sure what your fasting schedule is, but it’s do-able with IF.
I didn’t know low ferritin could cause weight gain. I don’t eat meat, and probably have some absorption issues, so I had a very low ferritin. I took prescribed replacement faithfully for years, and they changed the regimen a few times before getting me IV replacement. That got my levels back up.
I have had low iron in past so I make sure to include a small and regular amount of liver in my diet, during my eating times. I also regularly take a supplement. Not sure it is available in UK but I recommend one that doesn’t seem to give stomach issues —it is called bloodbuilder by megafood.