Air Canada's press release about the second daily YVR-LHR.
http://aircanada.mediaroom.com/index.php?s=43&item=935
Also - the second flight will be on the larger 787-9 (298 seats) not the 787-8 as was uploaded in the scheds a couple of days ago. The AC booking engine now reflects the 787-9.
So with the HD 77W and the 787-9, that's 748 seats per day or 5,236 seats per week on AC on YVR-LHR. That's a lot of seats to fill.
Comparing 2014 (when Virgin Atlantic still flew to YVR) with 2016 though, 2016 will still have slightly fewer seats per week on YVR -LHR. However, if you compare the entire London market (including LGW) between 2014 and 2016... Quite a different story thanks to Air Transat increases and the launch of Westjet to LGW:
YVR-LHR 2014 8,621 seats on 24 flights per week BA-12 AC-7 VS-5
YVR-LHR 2016 8,519 seats on 21 flights per week AC-14 BA-7
YVR-LON 2014 10,346 seats on 29 flights per week BA-12 AC-7 VS-5 TS-5
YVR-LON 2016 12,506 seats on 34 flights per week AC-14 BA-7 TS-7 WS-6
Included in this press release is a little blurb about YVR-BNE being moved forward two weeks to June 1st as well.