Agree - even though I know the Tube is direct, I find it a painful journey for the full hour to King's Cross St Pancras from Heathrow, so I usually get the HEX+Tube combo (or change at Hammersmith if I'm feeling brassic).

The high-speed service from St Pancras is excellent - very comfortable and very speedy. It's also an impressive station, with quite a lot to keep you going (not to mention good places to grab a take-away meal to eat onboard). Personally, for the sake of less than £20, I'd opt for the peace of mind of a flexible ticket. That way you can turn up, stretch your legs, get some food and a coffee, and hop on whatever train you feel like to Ashford - they run every 15 minutes, after all.