Heathrow to St Pancras

Don't take the Heathrow Express to Paddington Station. Instead take the Underground train from Heathrow direct to King Cross/St Pancaras Station via the Picadilly Line.
The London transport planner doesn't take into account the time you waste at Paddington Station waiting in line to get Underground tickets to St Pancras. As the Picadilly line is direct to the Chunnel train station at St Pancras it will be fastest (an cheapest) in the long run.

We did the reverse trip last month, Kings Cross/St.Pancras to Heathrow T5. It took just under an hour on the Tube via the Piccadilly Line, no change of trains. And it cost less than GBP6 per person.

To satisfy my curiosity, why didn't you fly to Paris either directly or via a connection at LHR?

We're returning to London a few days later for a conference and then catching the return flight home. The States to London fare was better than the States to Paris fare.

Barring a significant flight delay, the plan should work, but don't dawdle. You may need an hour or more to navigate the airport, an hour for the Tube ride, and don't forget a half-hour minimum check-in time for Eurostar security at St. Pancras station.

If you do have a little extra time in St-Pancras before going through the security gate for the Eurostar departures, there is a Marks and Spencer store centrally located where you can pick up a sandwich and drink, saving a few euros over what is available on the train. http://stpancras.com/media/218100/st_pancras_international_station_map_20140801.pdf

Oh, careful. I tried to do what Southam suggested, about a year ago, at about 6 am with my trusty cup of Caffe Nero coffee and was asked to pour it out because I was told "no drinks through security". I did so and then bought a replacement at the Caffe Nero inside.,

I don't think there would be any trouble with food, and maybe a screw top bottle would be OK, but they certainly did not like my coffee.