Pakistan took a principled stand not to recognize a country created by oppressing the native population of that land.Just like the refusal to recognize the apartheid regime of South Africa.
Just because it was a British mandate doesnt make it right for them to give it to European settlers, completely ignoring the wishes of the native people.
Its obvious that for British, it was like a conquered land, and its natives conquered hostile people to do with as they pleased, like war booty.
The proceedings of British legislature or League of nations is just whitewash.How can you decide the fate of that land without asking the native people who always lived there?What moral right did they have to make such declarations other then the right of might?
If the British truly believed in the equality of all people, they would never have done that.
Recognizing such a state will mean recognizing the hook&crook(mostly crook) methods used to create it and the oppression that was done to the natives in the process.