Heathrow or Gatwick depends on what carrier. Sas and Ba flies to Heathrow and Norwegian flies to Gatwick.
BA departs 19:20 back to Oslo. Sas departs 19:30 and Norwegian departs 20:00. So it's almost identical. Trying to plan it so I don't need to stress a lot on Sunday to get back to the airport.

I reckon it's about 1 hour traveling time to London from each airport. and 1 hour from Euston station to Northampton.

Bus from Heathrow to Northampton seems to take 2.5 hours and bus from gatwick seems to take 4 hours.
4 hours on a bus ain't gonna happen.