This is socially incorrect. The socially correct way of pouring tea is to put the milk in after the tea. Social correctness has traditionally had nothing whatever to do with reason, logic, or physics. In fact, in England it is generally considered socially incorrect to know stuff or think about things. It’s worth bearing this in mind when visiting.
The reason it was always considered "polite" to put the milk in the cup first, and then pour in the tea (contrary to the above quote) was simply that, when tea was first introduced into the UK, most pottery was not made to cope with boiling liquids, and a cup was liable to crack if you poured boiling water into it. Hence the "safe" thing to do was to pour the milk first, and then add the tea to it.