Londoners, it's all about the fluoride
I was once asked if brushing teeth is fine to do even without toothpaste. He further explained that since it is the brushing motion that dislodges the food particles and sweeps away bacteria in the oral cavity, then why do need toothpaste for? If it only were for having that minty fresh breath, then you can just use some mouthwash.
However, using toothpaste is also an essential part of oral hygiene. It supplies the needed fluoride that the teeth use for strength and the prevention of decay and caries. It can also re-mineralize certain areas of the teeth that have weakened due to decay.
Even during the 1960s, tap water in several parts of the world, including London and the US has undergone fluoridation, given low levels of fluorine in the water. According to the local dental associations, this augments the fluoride need of each person. It meets the daily allowance of fluorine in the body which is often lacking in the diet.
Unfortunately, several people are also opposed to this fluoridation, saying that there is no difference between the oral conditions of children using fluoride filled water and those without fluoride. In fact, there are also studies showing kids having dental fluorosis due to too much intake of fluorine. Dental fluorosis is a condition wherein the surfaces of the teeth weaken and get discoloured.
In conclusion, anything should be taken in moderation. Fluorine can be an enemy and a friend, depending on how it is used. It will just be up to you on how to deal with it.