Curious that Manchester Airport to Heathrow Airport is about 15 miles further (on land - source google maps) than Exeter Airport to London City Airport, but BA schedule the trip for 65 minutes, rather than the 80 minutes that Flybe are allowing. I would have thought that unless you were unlucky, the holding time around 8am into LHR would be greater than into LCY. That would suggest that quite a bit of extra contingency has been built into this service to try and ensure that it has a good chance of getting a reputation among frequent flyers as being very punctual.