Lokilou, that is really awful and negligent really. The NHS is soooooooo useless, I've lost all faith after what I've experienced the last few years.
Your ferritin should be over 100, preferably over 150 if you've been deficient for a while. To get it up there, you need pretty high doses of iron 3x/day (the dose is calculated by your weight and the type of iron).
Whilst iron can cause gastro upsets when people take it (usually constipation) there is a heme iron which is made in the US by Three Arrows called Simply Heme, which is absorbed through a totally different pathway to other non-heme iron and doesn't usually cause any gastro issues even at high doses. I've imported it many times and highly recommend - they ship to the UK.
Meanwhile, go onto Facebook and join the group there called The Iron Protocol. (Not the UK one, but the 'main' group. The UK mini group is rubbish and doesn't have the guides and help which the main group has.)
I used to have very heavy periods, where every time I had a wee I was basically weeing out massive clots. This has been the case all my life and I have a history of iron deficiency my entire life too, since I was turned away from giving blood when I was 18 due to being anemic. It was only when I got to peri-menopause and had problems sleeping that a sleep specialist told me I should get my ferritin over 100-150. I needed an infusion to do that (which I had to get privately in London at the London Iron Clinic because the NHS disagreed that I needed one). Almost immediately my periods were way way way lighter, less painful and more like everyone else's 'normal' periods. I then read this is very common.....
So I would really look into this...