I often get asked this question and the answer is you should put the milk in second.
Of course if you’re a real tea connoisseur then you shouldn’t add any milk at all. The Indians and Chinese are mortified that we insist on adding milk to our tea. There are both practical and etiquette reasons for adding the milk first. The practical reason is that if you haven’t made the pot of tea yourself, you’re not sure how long it has been steeped for, so it makes sense to pour a bit out first to judge its strength. Putting the milk in first would hinder this and make it impossible to judge the colour of the tea.
"The Indians and Chinese are mortified that we insist on adding milk to our tea."
If you’re being a bit more snobbish about it, years ago the aristocracy would use fine bone china that was very expensive and well made to drink their tea. These cups were fine to hold boiling hot tea. However their working class staff would use more crudely made clay mugs that were not like the mugs of today, which are properly finished and glazed. These mugs were a lot clunkier and would crack if you poured in boiling hot liquids, which is why the lower classes tended to put the milk in first to cool the tea down.
However, you should only use boiling water for black tea, and there is a school of thought that the water should be around 95 degrees C to be perfect. The same is not true for coffee, which should never be made with boiling water – it should be around 90 or 95 degrees C otherwise it will burn the coffee and produce a bitter taste.
Of course times have changed and most people now use teabags rather than making a pot of tea. But you can still tell someone’s background from the way they make their tea. Some people still put the milk in first even if they are using teabags. Having said that I’ve seen very privileged upper class people put the milk in first and those from a lower class putting the milk in last, so it’s not an acid test. It’s really for practical reasons, which is why however you look at it, you should always put the milk in second.