This business about "I want to be a citizen not a subject" really is a bit of a bore. If you're British,you're a British Citizen. The fact that the country has a hereditary monarchy is irrelevant. If the monarchy were to be abolished tomorrow they'd still be citizens - and if the monarchy were restored the next day, they'd still be citizens.

The same applies to Australians and Canadians and so forth. All citizens of their respective countries.

There are arguments for dispensing with the hereditary monarchy - but stuff about citizens and subjects are completely irrelevant and just get in the way of any sensible discussion.