I have a similar case coming up -- arriving at Heathrow at 4pm on a Wednesday and need to get to Cambridge by around 7pm. There are two of us.
Thoughts:
1) Picadilly -> Kings Cross -> Cambridge
2) HeX or Heathrow Connect -> Paddington -> Kings Cross -> Cambridge
3) National Express
4) Rent a car
4) has been looking a bit more attractive recently. Am I insane for thinking that?
Thanks for any and all advice.