Okay, I'm from the US, but have did do a fair bit of research on this when I was in school.
I the British system, if you are not a Royal (I'm not certain exactly how that's determined once you get past the Monarch's immediate family) or the holder of a title in the peerage you are a commoner. So Lady Diana Spencer was a commoner, "Lady" was just a courtesy title because her father was an earl. I believe that this also extends to spouses of nobles. So while the wife of an earl is referred to as a countess and typically referred to as Lady Title or the Countess of Title, she is still a commoner.
Elizabeth Bowes-Lyon was the youngest daughter of Claude George Bowes-Lyon and Cecilia Cavendish-Bentinck.
When she was born her father used the title Lord Glamis, as he was the eldest son of an earl. He was technically a commoner but permitted to use one of his father's lesser titles as a courtesy. His wife was known as Lady Glamis. At birth Elizabeth was known as The Hon Elizabeth Bowes-Lyon, and yes she was a commoner even though her grandfather was an earl.
When The Hon Elizabeth Bowes-Lyon was three, her father's father died. Her father became the Earl of Strathmore and Kinghorne, he was now a nobleman. Her mother became known as the Countess of Strathmore and Kinghorne, she was still a commoner. Her eldest brother become Lord Glamis, he was still a commoner. Elizabeth herself became Lady Elizabeth Bowes-Lyon, she was still a commoner.
Then when Lady Elizabeth was 22, she married Prince Albert, the Duke of York. She then became the Duchess of York and was no longer a commoner, but a royal. Her children were at birth Princess Elizabeth (now Queen Elizabeth II) and Princess Margaret.
When Prince Albert became King George VI, the Duchess of York became Queen Elizabeth, the queen consort. She did not get a numeral because she was not the monarch.