Doing some rough calculations it looks like you would be up for around AUD$3,800 if you were successful in getting all 4 segments upgraded on a SYD/MEL/BNE-LHR return fare. Add your $2,000 economy ticket on top of that and you're at $5,800.
Thai offers J class tickets to London for around $7,300 normally, so paying on check-in will save you about 20% off a pre-purchased business class fare.
For some people this will be a bargain, for others the uncertainty of the upgrade might outweigh the potential saving!