You cannot combine a revenue and award booking, and one way revenue TATL fares are very expensive so I would forget about that option.
You can combine revenue WT one way with revenue WTP on the other.
However note that there needs to be award availability in CW to upgrade from WTP to CW with avios. There is currently no availability in either direction on LHR-DEN in May so you will not be able to upgrade at present. Award availability may or may not become available at some point between now and the flight.
Ok can I book wt out and wt+ return now and if CW becomes available upgrade the return?