It is impossible to know why and when the British started putting milk in their tea. It seems to have started in the days when more green tea was being consumed than black and perhaps milk helped to soften the bitter astringent flavor; or, maybe it was the result of contact by merchants with the Mongolians or the early Manchus who also put milk in their tea. Or was a little milk poured into the Chinese tea bowls used in the seventeenth and eighteenth centuries before the hot tea in order to reduce the risk of shattering the fine porcelain? Thomas Garraway's advertizing broadsheet of 1660 clearly claimed that tea" 'being prepared with Milk and Water, strengtheneth the inward Parts," so right from the very first days of tea-drinking in Britain, milk was an option and in the mid-eighteenth century it became fashionable to take milk in tea. However, the Dutch had exactly the same contact with the same traders but did not use milk. A Dutch traveler, Jean Nieuhoff, experienced milk in tea at a banquet given by the Chinese emperor for the Dutch ambassador and his staff in 1655, but still the Dutch did not start taking milk in their tea. The French, similarly, did not show any preference for tea with milk and Marquise de la Sabliere seems to have been alone in her apparent taste for it in 1680.
The custom of adding milk spread the length and breadth of Britain by the end of the seventeenth century, and subsequently traveled to the British colonies. Today, the majority of blends created for the British market are designed to be drunk with milk and producing countries bear this in mind when manufacturing teas for export to Britain. However, the addition of milk to a cup tea is a purely personal choice, although drinkers should be aware that milk severely spoils the flavor of some teas - notably all white and green teas, pouchongs, and oolongs, most China blacks (with the exception of Yunnan), first flush Darjeeling, scented teas, and some lighter black teas.
Should the milk go into the cup before or after the tea? Tradition insists that it should go first (as previously mentioned, originally to protect the fine porcelain tea bowls). Certainly, tea poured into milk enables the milk to mix in better. The official scientific view is that it is better to pour milk into the cup first because it then cools the first tea that is poured in, thus reducing the risk of scalding the fat in the milk - which can cause an unpleasant taste. Other people, though, prefer to add their milk afterward, claiming that it makes it easier to achieve the desired proportions of the two liquids. However there are no hard and fast rules, it is a matter of personal preference, and no doubt the debate will run and run.