I can see you getting 5 answers to this, all correct in the opinion of the poster, with vociferous arguments developing beneath each ;)
So, let me try pre-empt them all.
Traditional, traditional, historical origins.
Loose tea; tea pot.
First warm the pot with about a cupful of boiling water & discard.
One teaspoon* of loose leaves per cup, plus 'one for the pot'.
Add boiling water sufficient for your cup count. Allow to steep for four minutes before serving.
Upper class variation
Pour through a strainer into each cup and serve, to which each guest adds milk & sugar to taste.
Middle class variation
Milk goes in the cup first, poured by the server not the guest.
This is because very expensive porcelain is really not subject to the disparities of heat expansion of cheaper porcelain [china] which could crack if very hot tea was added, so the server pre-empts the possibility. This is also the origin of the 'teaspoon in cup as you pour' affectation.
One of the sad and sorry lessons of the British middle classes of the 20s to 50s.
Working class variation.
Milk last, probably straight from the bottle.
No-one could afford china, and cheap pottery [stoneware, ironware, earthenware etc] is immune to being cracked by a bit of hot water.
Tea first or milk first has been a source of heated debate for a century or more, though actually it was only the middle classes who were initially afflicted/affected.
Modern version.
Tea bag in mug. Then either…
a) Add boiling water. Steep 4 minutes, squeeze & discard the bag. Add milk & sugar.
b) Add milk, then boiling water. Steep & keep stirring, squeezing the bag repeatedly against the side of the mug as long as you have patience for, or until it looks about right.
Modern pottery, of course, is far less affected by heat as its Victorian counterpart, so the choice is now one of education/upbringing rather than necessity.
This is subject to the historical prejudices of the generation before last's grandparents' version of any of the above choices.
The debate continues, milk first or last.
The answer, as it has always been… is last, unless you have cheap china or no patience.
Let the comments begin ;)
*One thing to note is that originally, the teaspoon used to measure the tea was larger than the one for used by guests for sugar & used to stir. This may be the origin of the 'one for the pot' as we tend to only own one size of teaspoon these days.
Note 2: I'm referring to Indian 'black' tea, which only became popular in the UK around the end of the 19th century. Assam/Ceylon tea was only grown commercially for the first time in around 1870. Before that, there was Chinese green tea - which had almost vanished by around 1900 in the UK. This cultural and socio/econmic structure was therefore almost entirely developed and engendered in the first half of the 20th century; along with the rise of the middle classes.