How do you sell SAAS products overseas and handle tax?

Can I sell a product to Aussies and Americans and just pay taxes in UK?

Yes, in your case you will most likely only have to pay taxes in the UK on Australian and American profits, unless you have physical operations in those countries that generate profit. On EU profits on the other hand, you have to deal with the maze of VAT for each customer. If you were to sell physical products, then it's different story.

Someone in WIP has recommended Paddle as they cover taxes. I did more googling and found out for Australia you pay 10% GST once your sales get to a certain point like $75000 or something works on top of stripe and takes care of VAT moss and all those taxes...

I've decided to go with to circumvent this problem, they handle VAT.