When cotton was first brought into the UK they did not know how to spin it, so they spun it like wool (hence where the name cotton wool comes from). By the 1830's the UK had become the richest country on the planet thanks to its sale of cotton.
Long work days were endured by the workers in these harsh times, sometimes working 12-14 hours a day. In due course the 'tea break' was introduced into the daily routine where the workers could have a refreshing hot drink that was also healthy (thanks to the boiling water) and, with the added sugar, it gave them energy.
An initial problem at the time was that the cheap pottery which was used for cups often broke because it could not handle the heat of direct boiling water. Hence, it became commonplace to add a dash of milk into the vessel first to stop this happening.