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.
To avoid conflicting traffic flows at both Heathrow and City your Dash is going to have a choice of lovely detours depending on runway direction and prevailing weather
The Northern route over Oxfordshire, Buckinghamshire passing just south of Luton towards St Albans Hatfield Cheshunt and entering the circuit for City around Romford/Upminster
The Southern route towards Southampton through Gatwick area traffic around Midhurst up towards Biggin entering the southern pattern around Bexley/Dartford.
No way straight across the city other than transiting at above 14,000 feet towards Sheerness and backtracking into the circuit towards Dartford
Air mileage probably greater in reality.
As to the Manchester shuttles two circuits of Bovingdon with fellow shuttles and combined with European Lambourn traffic filtering out Heavies and hold times can be pretty short.
For optimal separation purposes ATC operate a policy of grouping Heavies where ever possible at Heathrow not all slots are equal !