I travel to the USA once a year, and to the UK 3 to 4 times a year. Apple products in the USA are around 10 - 15 % cheaper than here - that is, if you DON'T declare it and pay VAT like you are meant to when you arrive back in the country. If you declare it when you return, and pay VAT, then the pricing is about the same.
For the UK, because they have such a high VAT rate (20 % !!!!!!!), I found it slightly more expensive to buy Apple products in the UK - you can see for yourself - go to apple.co.uk, and get the price and multiply by 18.4 !!!!! you will probably find it is more expensive there after our weak rand exchange is factored in and their high 20 % VAT included.
If you still want to buy an Apple product in the UK, and want to claim the VAT back, you need to get a "VAT 407" form from the Apple store. Take this completed VAT 407 form, together with invoice and Apple product STILL IN IT'S BOX, and show it to the VAT refund guys at Heathrow airport. If they are happy, they will stamp your 407 form. Now the tricky part - if you want a FULL refund, you have to post the now stamped 407 form back to the dealer that you bought it from, together with your C/C number or bank details, into which they must transfer the VAT money back to you. Ye.... I know, you will be back in S.A and wondering if it will ever get paid back....
If you want CASH back for you VAT claim, you take the stamped VAT 407 form through security at Heathrow. Once you are through security you have to go to the Travelex people that exchange money. Hand them the 407 form, and they will give you cash. HOWEVER, these guys are sharks. They charge you a commission. The last time I did it, they charged me almost 40 % of what I was supposed to get back. So I lost half my VAT claim to the Travelex sharks. After travelling back and forth so many times, I have come to the conclusion that it's not worth the bother of buying there in the UK. You might be worse off or break even at best. It's too much hassle, and you are not going to really benefit. AND, if they stop you at O.R.T when you arrive back in S.A, and search your suitcase, and find your goods.....the S.A customs will charge you 14 %......which is why you could be worse off than buying locally.
I don't know about Australia, as I don't travel there.