Abomination, without question
To put milk in the pot (debates on how much that affects the brewing process aside) is to assume everyone wants the same tea:milk ratio which is unbelievable arrogance. My view is a splash of milk to give it a colour resembling a manilla envelope, my mother on the other hand prefers what is basically a glass of milk that's been in the same room as a teabag for no more than 4 seconds.
Interestingly, I recall this debated on radio once - Chris Evans once argued (on the R1 breakfast show I think??) that if you put milk/sugar in coffee or tea, then you don't like tea.
Equally, Frank Skinner recalled a tale of someone's response to an adult requesting sugar in their tea - "what are you, 12??"