Official government advice to pregnant women and new parents in the UK is not a great example of effective communication. As someone in the UK and of an age where my close circle includes several young children and a few more current pregnancies, I wouldn't be surprised if many parents had never even heard of that scheme or who qualifies for it, nor the accompanying advice on vitamins for young children for that matter. As I understand it, right now you don't even get a lot of the usual face-to-face check-ups during pregnancy or early life where such things might be discussed. So as you mentioned in another comment, the first problem here is getting useful information to those who would benefit from it to begin with.

We just got given it whenever we took our daughter to get weighed and checked over during the first year of her life. You only need a few drops so the couple of bottles we got given lasted ages. Maybe that’s specific to London? Was that not your experience?

I've asked around a little this morning, and so far 0% of the parents of young children I know were aware of this. :-(

Plenty of them had been going to those weigh-in sessions before they stopped because of the virus, so apparently it isn't a universal thing.

Several were not aware of the advice about vitamin supplements either, so this discussion has had at least one benefit because now everyone is looking that up!