It's a class thing. Originally tea was so expensive only the rich could afford it. They used porcelain cups which could take the thermal shock of near boiling tea and then they would add lemon. When the price fell far enough for the workers to afford it employers encouraged their workers to have tea breaks instead of drinking gin. However they could only afford earthenware cups which would crack if hot tea was poured into them. To prevent this a small amount of milk was put in the cup first. This avoided the thermal shock.