Don't think there's any problem with importing seeds from Australia; live plants or sending stuff the other way is another matter. DEFRA site is best place to check, but it's down at the moment. Chiltern Seeds offer some of the species you mention, though.
I've grown a number of Australian plants from seed -Callistemon, Grevillea, Anigozanthos, Acacia, Eucalyptus, but none of the ones you mention.
The waratah has a reputation for being quite tricky, though - very prone to damping-off, for one thing.