Don't do it, simply too much time on a plane and the vibrations eventually seep into your body. The longest stretch I've flown is LIS-LHR-NRT-HKG-LAX-ORD with <3 hours between flights, and it was getting intolerable (despite being in F with sleeper seats for the middle 3 legs). There really is a limit to how many hours you can spend on a plane before your body rebels. You should be able to manage DXB-LHR-SYD though, so doing an overnight at DXB instead of LHR is an option.