The UK is just one country.
It is just in this country - and maybe a couple of others - where breastfeeding is delineated along class lines. Yes, it's true that here in Britain breastfeeding is more likely amongst older, better educated middle class women.
But, with all due respect, Britain is not the centre of the universe.
Plus, it has a massively entrenched formula feeding culture. Breastfeeding is NOT the norm here.
The are other countries which have a far better breastfeeding culture - Scandanavian ones, Antipodean ones, African and Asian ones - and the benefits of breastfeeding are there for all, regardless of class, since so many more women breast feed.
Also, this is just one point in history. Back in the past, breast feeding was obviously much more widespread, because there was no alternative. Again, the benefits of breastfeeding were there for all to avail themselves of, since so many more women breast fed.
The conclusion that 'breast fed babies are more intelligent because their mothers are more intelligent' stands up only if you just look at Britain, and further, if you just look at Britain at this point in history.
So in other words, it doesn't stand up at all.