It's a complicated one and tbf, I had to go and look it up... but basically it seems to be.

There are 6 different types of British nationality. These are:

British citizenship

British overseas territories citizen

British overseas citizen

British subject

British national (overseas)

British protected person

Until 1949, nearly everyone with a close connection to the United Kingdom was called a ‘British subject’.All citizens of Commonwealth countries were collectively referred to as ‘British subjects’ until January 1983.

From 1949, you automatically became a British Citizen if you were born to people who qualified to be British Citizens under the law of that time. Most people in the commonwealth remained 'British Subjects'. That can cause a lot of problems when people came and raised families here but didn't realise they weren't citizens... (See the Windrush scandal) we've screwed over a whole bunch of people on that technicality (Joanna Lumley and the Ghurkas etc)

I suspect all citizens are her majesty's subjects - but I don't have to like it and I don't have to play along. If needs be I'd register myself as a moveable republic with a border at my waterproof feathers :o)