GCM is a notoriously expensive route in my opinion. It is often a very busy route as only oparates 4 times per week, and is rammed from NAS to LHR.

I would be tempted to book MIA-LHR-MAN rtn on one ticket, and use avios for the GCM-MIA rtn on another.......a snip at only 9000 avios before it goes up n Feb. In addition if you know you were needing to make any other similar trips well in advance, I would personally do a one way avios and nest tickets from there onwards as is usually much cheaper with an ex-UK (or even better ex-eu if you can) than ex-GCM.