It doesn't add up, if stronger demand was there once bmi went away, Transat or Westjet would have mopped it up. They didn't. The Rouge service was a summer peak seasonal leisure service, so there's a different driver at work in market. BA's BAU on LHR-YYZ was 2 x daily with a LGW leisure summer service pre COVID, Air Canada were four daily at LHR with Rouge at LGW one year too. That's a very strong London offering, but anyone from the Manchester area has a BA connection option but Air Canada was cut off from the local market. I suppose they might be toe dipping to see if more than a summer option might work? Almost no one is connecting MAN-LHR-YYZ on Air Canada though, and FRA is a good option if you need to connect I suppose. Same with United, both walked away.
BTW those nos show a 13% increase in 32 years, that's underperforming massively vs. wider market.