You can own CDIs or Shares, you can trade CDIs on ASX, or shares on LSE.
You can typically convert your CDIs to shares via your company share registry. If you convert them, you need to have the ability to trade on the LSE - that means you need a brokerage account for international trading, or a UK brokerage and the ability to transfer funds to/from it.
The market is made by a market maker - as mentioned above who purchased 1.8 million shares for A$0.2889 They then 'make the market' on the LSE. until such time as it is self sustaining.
The total number of CDIs + Shares is consistent, they must be converted on a 1 to 1 ratio in the case of PM8 - this isn't always the case, ratios can be different.
There is the possibility of arbitrage as the red tape creates inefficiencies that can be exploited by people who are motivated to do so. Generally you would expect the value of shares and CDIs to be pretty close factoring in FOREX movements and risk, due to people happy to go to the effort for the arbitrage opportunity.