These are British born citizens. They are natural citizens. The government has to let them back in.
No. they can pass a law making terrorists persona non gratis. As thinga stand now they have to take them back. It needn’t stay that way.
Not so much, by joining a foreign military and taking up arms against the UK they have effectively taken an affirmative action to renounce their citizenship.
Under UK and American law the government can not strip you of your citizenship but you can loose it through your own actions.