Ground Transport in London from Heathrow isn't bad, if you can afford the prices. But to the west, north and south they are crap.
That's why National Express do so well.
When I lived in Northamptonshire, in an area not served by National Express, to get to/from LHR was a public transport nightmare, it would have meant a 1 hour bus through the country lanes and then a walk to the train station, then a train in to one London terminus ... and you know the rest.
That's when I started utilising one-way hire cars, cheaper and more convenient than public transport and cheaper on car parking (i.e. no car parking) than using one's own car.