Unfortunately no - with Star you must cross both the Pacific and Atlantic in each itinerary, but only once each time. You would have to do something like MEX-LAX-NRT/SEL-LHR, but then still get back to CAI as well. Your itinerary must start and end in the same country on *RTW's. Nothing to stop you ticketing (in your itinerary above) LHR-CAI (I know a * carrier doesn't serve the pair, but work with me on it - it's an example!!) and never using the LHR-CAI portion, but it must be ticketed as such. If you could move around your Euro and Sth Am segments a bit more, you'd probably squeeze under the Star2 limit by going back via the Far East, but it's still touch and go - the long backtracks to THR are a killer.
Cheers.