Sorry this is what Which say about Follow On Milk:
"What about follow-on formula?
Follow on formula can be used from six months (it is not suitable from birth). All the brands have a follow on formula and these are usually labelled as Stage 3, other than SMA follow on formula which is labelled as Stage 2.
Follow on formula contains the same ingredients as infant formula and the levels of nutrients it contains are also strictly controlled under the Infant Formula and Follow on Formula Regulations 2007. However, it does contain higher levels of iron and vitamin D.
The restrictions that cover infant formula also cover follow on formula except that follow on formula can be advertised and offered on promotion. 67% of parents told us they have used follow on formula.
Does my baby need follow-on formula?
No. The government advice is that follow on formula is not necessary; you can continue to give infant formula if not breastfeeding.
As such follow on formula can not be purchased with Healthy Start vouchers. However, some retailers do offer discounts or money-off vouchers.
Babies are born with an iron store that by six months has started to deplete and so they need a higher dietary intake. Because of this some people choose to switch to follow on formula, however when weaning your baby you can start to include iron-rich foods such as fortified cereal, red meat and green leafy veg into their diet which will be enough to meet their iron requirements.
The government recommends that all children from six months to five years should take a daily supplement that contains vitamin D. However, if your baby is drinking around 500ml of infant formula a day then they will be receiving enough vitamin D to meet their needs so they won’t need additional drops."