Wouldn't risk going into London with only six hours between flights & I certainly wouldn't recommend the HEX or Heathrow Connect to do it with. Both take you to Paddington which is a long way from any of the likely things you'd want to see. Once you've worked out the total travel times you find that you are spending a lot of money to gain 10-15 minutes at most in TOTAL and in some cases it's quicker overall by Tube.

If you want to get out of the airport, get a bus to Windsor BUT allow a lot of time to get back