Hope someone can offer advice? I've published 3 books, and currently have them for sale in UK, and USA, in terms of other English speaking countries, eg Australia, would I need to complete any sort of tax form like I have for America? or just declare my foreign income annually as I do now?
No. If you are a "U.S. person," you get whatever you can extract from Australia (or other places) and it will show on your annual 1099 from Amazon. That's the only form you need to worry about. You do not fill out tax info for foreign countries.
That said, if you start making money hand-over-fist, you may need to begin paying estimated taxes during the year, but that's another story.
Im in the US and have sold books in the UK, Germany, France, Japan, Australia, and India. Amazon is the seller so they handle the taxes. They pay royalties as a US company to a US citizen regardless of where the book was sold, so the taxes are pretty simple. I would assume it's similar if you're in another country. One form to receive royalties.
Why just publish in English speaking countries? There are native English speakers elsewhere. I sell a few in Spain and Germany, mostly I'm guessing to British ex-pats. It costs nothing to tick all the boxes, setting a sensible price in the local currency, such €2.99 or whatever.
If you chose worldwide, you're already publishing everywhere. If you truly chose to publish only in the U.K.and U.S., that was not a wise choice. You can easily correct your mistake by updating and republishing.