OK - here's the gen on formula and different formulations.
'Infant formula' in the UK is formula intended for babies under 6 months. It is basically skimmed cows milk with fats from other vegetable or animal sources added in, plus a whole raft of additions such as nucleotides, vitamins, prebiotics again from vegetable or animal sources. The protein in the cows milk has been 'modified' (this is the technical term) with the aim of making it less hard work for the baby to metabolise.
In the case of 'stage one' milks (different brands have different terms for this) usually marketed as being 'for babies from birth or when breastfeeding is being supplemented' or 'when moving on from breastfeeding' or some such, these proteins have been modified to make them predominantly whey and this mirrors the composition of human milk where the protein is predominantly whey (remember this is cows milk we are talking about, so in my book this doesn't make it 'close' to breastmilk).
In the case of 'stage 2' milks or 'milk for hungrier babies' or whatever term the manufacturer is using, the proteins have been modified a bit, but the predominant protein is casein which is 'tough' and takes longer for the baby to break down....hence the idea (unresearched in any proper study) that babies are more 'satisfied' by it.
Both whey-based and casein-based formulas are able to be marketed for newborns, but the preference in the UK is for babies to start off on the whey-based and then typically to move to casein-based, often when the mother feels the baby is feeding too often or is not satisfied.
This as far as I can see is marketeers catering to a market - a market of mothers who feel their existing formula is not keeping their baby 'full' - and the mothers may be right in thinking that, of course. Casein based formula has been around longer than whey based, and manufacturers needed to keep the casein formula rather than replace it entirely with the apparently more breastmilk like whey based.
The calorie content of these milks is more or less the same, by the way.
All the UK-sold formula (as far as I am aware) has a whey-based and a casein-based version, so it's nothing to do with a particular brand. For example, SMA Gold and White; Aptamil and Milumil. Mothers are not told much about this. Instead they are given marketing speak like 'gentle on baby's stomach' or 'for hungrier babies' .
Follow on formula (note they don't use the word 'infant') began when restrictions were introduced on the way formula could be advertised and marketed.....manufacturers developed a new product deliberately not targetted at 'infants', but at babies over six months, which they could brand, package and market as they wished, keeping their logo and their name in front of British mothers. Again, I think all the manufacturers have a follow on formula.
The main difference in follow on is its relatively high iron content compared with ordinary cows milk - it has a higher iron content than infant formula, but that does not mean it is 'better' than infant formula. Much of the iron content in follow on is excreted because there is far more than the baby can process. This is why it is very common for babies and toddlers on follow on to become constipated, and the usual advice for them is simply to go back to infant formula, which is, in any case recommended for non-breastfed babies until 12 months (or longer - 12 months is the time when current guidance says cows milk is ok for a main drink).
Hana: breastmilk does change as babies grow - but not in the way that formula changes. Breastmilk doesn't acquire more casein, or more iron - which is the way formula changes.
Formula changes for marketing reasons, not to mirror breastmilk changes.
Sorry this is long but no one else is telling mothers this from what I can see :)