Your sums may be a little out. MFU from WT+ to CW, one way LHR to YVR costs 12,500 miles. That's correct.
One way, LHR to YVR it earns 4,713 miles. There's a cabin bonus of 25% for having booked into WT+, which is a further 1178, bringing the total to 5,891. There are also status bonuses if you are Silver or Gold, but by the sound of it you are blue.