A single toilet is common in the U.S as well, I have never heard of it being used as an excuse for Americans to defecate in public.
You should rethink that excuse, row houses in London sound pretty normal.
Having single toilet is adequate for most small families who can easily stagger toilet usage in the morning rush hour. Singles, couples, retired folks, rural folks, people who do not have to use toilets all at once to get ready for job commute in the morning would have no need or excuse to take desperate measures. That is normal. But we are not talking ‘normal’.
When shit happens, it happens, it can not wait and it does not need ‘excuse’. Either its in the toilet or it is in the pants or somewhere else. And London row houses does create situations that are not ‘normal’
In USA, size of families tend to be smaller, with sizable population consisting of singles, single parents, retired folks and couples with no children. There is also sizable population who does not have to get ready in the morning or go out to work in the morning. For them, having only 1 toilet is never an issue. Most 2-3 bedroom apartments/houses have more than one toilets anyway. Most landlords also rent 1 bedroom/toilet apartments only to smaller families. Plus, America has invested heavily in creating ample public toilet/restroom facilities and created building codes to mandate toilets in all public places, which mitigate any need or excuse to go out in public.