Victorian era: Who decided which countries they were about to conquer?

Most of the British Empire was in place by the time Victoria came to the throne, so they didn't conquer much of anything.

While the Queen had some influence over foreign policy, most of it was determined by whichever government party was in power, through their Prime Minister.

Go read Winston Churchill's History of the English-Speaking Peoples. It's great reading and you'll learn all this stuff so it makes some sense.

Which ever country had something they wanted. No other reason was needed and it didn't matter who signed the orders.