I know brush cleaner was available at the Singapore duty free stores, so its plausible the Australian ones have it too, but there are a few catches. Firstly, I'd say the bottle of brush cleaner is larger than the liquid limit, and I wouldn't expect that Mac would be able to seal it in a tamper proof bag, so it wouldn't be allowed on if you needed to transfer. Secondly, brush cleaner is marked as a flammable good due to all of the alcohol in it, so its prohibited from flying as well anyway.
As prices go, the UK is generally cheaper than Australia if the exchange rate is good; also check the exchange rate and prices if you're transferring in Asia as it can often be cheaper again there. I just got the tax-included price from the Mac websites of the countries visited and used a currency converter. I had my brother pick some stuff up for me in Korea on the way to the UK because it was cheapest there, but it appears that the exchange rate has inverted and the UK is cheaper at the moment. The Australian duty free stores sell the stock at a flat 20% off the domestic price, which is an okay discount, but there are even cheaper ways to get it.