Will try to answer the Qs: the only real research about UK babies and toddlers risking vitamin deficiency is with Vitamin D (you can google if you want, or do a PubMed search and see the medical papers on it. The concern has been with cases of rickets and with Vit D deficiency found on testing. At the same time, there is growing concern that all of us might be going a bit short of it, especially if we live in the North of Europe).
Breastfed babies of normally nourished mothers do not need supplements - there is, of course, Vitamin D in breastmilk.
Beyond 6 mths, babies start to need more Vitamin D and this is naturally enough supplied in the diet - breastmilk plus whatever solids they are having - and from sunlight.
FF babies get extra Vit D in the supplements already added to formula. A white bf baby who is outside most days, bf liberally and moving onto solids, is highly unlikely to need supps, but if they are dark skinned (and therefore need more light to make Vit D) , they may not get outside enough, esp in winter, esp in the North...a nutrition-minded HV should help individuals make these decisions, and when it would continue to be of benefit regarding the age of the child.
You can buy kids vits, and I think they are also available through Healthy Start, but I am not sure about this these days.
I am highly sceptical about Growing up milks. They prob do have added vit d, but they are an expensive, processed product that is there to hook consumers into the formula market. Just my opinion!
Minadex has been around for ages and ages - it's not very expensive but there are probably unbranded 'tonics' for kids that are cheaper. I would guess it has Vit D in it - read the label or the chemist. It's probably quite pleasant to take (check for the sugar content though. It's a syrup, so probably high in glucose/ sucrose) and you can mix it in other foods - but of course you can with kids supps which are usually drops.