1. Grammer looks fine to me.

2. The plural form of English is Britons as far as my writing is concerned. Yeah I know, don't care.

3. People and They are your (pro)nouns. English and Australian are your adjectives.

If, for whatever reason, you wished to change forms so you could use Australians, consider:

Those people aren't English. They're Australians.

It would still be grammatically correct. It just lacks the balanced style of:

Those people aren't Britons. They're Australians.

There simply isn't a suffix you can put on the word English to transform it the way you do to Australia.

1. Grammar looks fine to me.

2. The plural form of English is Britons as far as my writing is concerned. Yeah I know, don't care.

3. People and They are your (pro)nouns. English and Australian are your adjectives.

If, for whatever reason, you wished to change forms so you could use Australians, consider:

Those people aren't English. They're Australians.

It would still be grammatically correct. It just lacks the balanced style of:

Those people aren't Britons. They're Australians.

There simply isn't a suffix you can put on the word English to transform it the way you do to Australia.