For NatEx, if they don't serve MK, they will serve generally Finchley Road or Golders Green which forces them down the M1 and then using Finchley Road into London. If none of these places are served en route, in many cases M1 and Finchley Road is the most direct route. Overnight services heading north tend to still have Finchley Road, Golders Green, MK. Some serve Heathrow granted but most are still somewhat forced to use Finchley Road and the M1 corridor in some way. Whether that be because of stops in London or stops nearby.
Megabus & Flixbus has been answered above by JonathanH
Reduce the number of junctions and where there are junctions, try to make it so that conflicting movements are kept to a minimal which then means more flow and more 'green time'. Do what the rest of London is doing and try to deter people from driving by blocking off junctions and forcing them to go through more side roads. Remove car parking and loading bays from bus lanes and better enforcement of red routes since the bus lanes are pretty useless on much of Finchley Road thanks to the parking, deliveries, the 'Ill only be 2 minutes' squad. You go on the A40 or A4, if there is parking, it tends to be off the main traffic lanes in laybys or in side roads, that is how Finchley Road should be. Without a complete rebuilt, it will unlikely ever be perfect, but there are steps that can be taken to improve the situation. TFL/councils seem intent on slowing everyone and everything down though.
Travelling from the M40 or M4 is so much better, you feel like you are getting somewhere rather than Finchley Road, as it's been said, it's an hour from leaving the motorway to getting to the coach station.