I had a quote for 7.5" discs to be shipped over. They were 23kg. And so I was quoted £250.

Needless to say, I didn't go ahead with it... Given the size of the package, it seems a bit rich.

I had air springs (not for Mini) sent from the US to me for 'only' $57.

They were much lighter, but the boxes weren't smaller. But UK Postage is generally dearer than postage from the US.
If I were buying ex-UK, especially if I were buying 8.4" discs, I'd be buying new, from
Hythe Performance. They advertise in MiniWorld, etc. And look at the price compared to
Mini Spares for the same kit - over £100 cheaper!

Remember, we don't pay VAT. So that's only A$700 for the brakes, hubs, cv's, all new.

Just remember there'll be a significant amount of postage on top of that... I've bought parts from Hythe before, they were good to deal with.