Eurostar from Heathrow airport

If it is not too late, consider taking a connecting flight instead. No need to deal with British immigration etc,

As Marco is saying, the Eurostar is the best way to get from city center to city center, but is not so great if you are already at Heathrow. From there, it's easier to take another flight (unless, of course, you've already purchased your Eurostar ticket).

Thanks for the responses...we are flying into Heathrow airport. I will check into airfares but we like trains even if they are more hastle like in this case.

If you have already booked a standalone flight into Heathrow you have already eroded one of the advantages of a connecting flight on a single ticket - ie being protected onwards if your inbound flight is late.

If changing to the Eurostar you need to allow for the imponderables of the length of time to pass through immigration (although there aren't any regular issues with this at the moment), the length of time to get from Heathrow to St Pancras (usually around a minimum of an hour on the Piccadilly Line if there aren't any delays, taking into account negotiation between stations), and needing to present yourself at Eurostar check-in a minimum of 30 mins before departure. Any checked-in bags on the plane would be another wait.

If you arrive late at check-in your tickets are worthless unless you have paid for the more expensive flexible type.

You do miss out the airport stuff at the Paris end, although you would have the option of arriving at Orly rather than CDG.

It's apparent you wish to travel by train to Paris after landing at Heathrow.

In answer to your question please keep in mind the following. 1.) Allow a minimum of 3-4 hours to get from point A to point B taking in consideration customs, is the flight on time and what means you finally choose to get to St. Pancras Train Station. 2.) In order to get the least expensive rail tickets you should be booking 3 months in advance and understand it's an ABSOLUTE that you must be at St Pancras a minimum of 30 minutes before departure. Might consider paying more and getting a flex ticket in case you miss your train.

Keeping all that in mind your options are:

1. Take the Tube (London Underground) from Heathrow directly to St. Pancras on the Piccadilly Line. About an hour's journey. CHEAPEST way. Uncertain if you buy a one day travel card which is $12.00 or what the cost of a single one way fare is these day. You'd have to ask at the station in whichever Terminal you arrived at.

2. Taking a Black Cab which would run you over a 100 pounds (about $150) and given traffic conditions could take an hour or more. Or Try Uber which is half the price BUT with drivers who don't know the back roads or how to get around traffic congestion and get you to your locale in a timely fashion.

3. Pre book a car hire around 50 pounds ( about $75). Same deal. Have to keep in mind traffic congestion.

4. Take the Heathrow Connect or Heathrow Express trains from Heathrow to Paddington Station and then a cab from Paddington to St. Pancras. 30 minutes on the trains to Paddington and 15-30 minutes in the cab from Paddington to St. Pancras. Express fares $33/person. $16.00/person for the Heathrow Connect. Then add on the cost of the taxi.

5. Bus. $10/person ? You'd have to check on frequency of busses for that route and with all auto transport keep in mind traffic congestion so a minimum of an hour.

Can be a smooth transfer from the airport to the train station or hell on earth. Need to be prepared for both.

There are no realistic bus routes these days from Heathrow to St Pancras unless you want to change about 5 times and have over 2 hours to spare.

Rather than rush over London, why not stay somewhere overnight, keep your blood pressure down and recharge your batteries and go off the following morning. After all if you'd wanted to just go to Paris you would have flown there ....

Full-fare 2nd class tickets for Eurostar trains cost $300 per person! Cheaper tickets go on sale months in advance for as low as $65, but do not offer any refund if you miss the train, so you must allow time for delays as well as transportation, same as if you were changing to a budget flight (not code-shared with your in-bound flight).

Traveling from Washington, DC to Paris is something that my wife and I do every year. Usually, we spend 2 days in London before taking the Eurostar to Paris, but we've skipped London twice. Three years ago we took the tube from Heathrow to St. Pancras Station. Our British Airways flight arrived at 0930, so I figured 1 hour to clear customs and immigration, and another hour for the tube to get us to St. Pancras. I had the choice of a 1200 or 1400 departure on the Eurostar. Not wanting to miss the 1200 train, I selected the 1400 train. Of course, we arrived at St. Pancras at 1115, and had to sit there for an extra 2 hours. But I'm sure that if I had selected the 1200 train, that something would have happened, and we would have missed it.
Last year, we changed planes at Heathrow, and flew to Paris. It was the pits. We got off our flight from Dulles, and although we didn't have to go through the immigration line, we did have to go through security all over again. Then a 2-hour wait for our connecting flight to CDG (Airbus A-320). Never again.

We did this transfer March 28. I booked non refundable, changeable Eurostar tickets and selected the 12:30 pm train with a scheduled 7:30 am arrival at Heathrow. I did not want to miss it. Our flight arrived early, customs went reasonably smoothly. We stopped at the London transport office in the airport and got Oyster cards for the five of us, including a youth discount on the cards for my 15 year olds. We planned to return to London later in the week.

We rode the Picadilly line to St Pancras station. It took an hour. We managed with our carry on luggage. St. Pancras station is beautiful with lots of interesting shops, etc except it is open air and my jet lagged teens were freezing. They have a left luggage station but they wanted 6 GBP per bag and my husband and I were too cheap to pay it. I took the most energetic teen to the British Library which is a very short walk away while the others stayed with the luggage. We actually had to wait 15 minutes for it to open at 10 am. We loved the British Museum. Great artifacts. After about 45 minutes we walked back and gave my husband a chance to go. The other teens were on strike. When he returned we took all our luggage across the street to the Burger King and ate lunch in the warm before going back just in time to catch the train. Everyone napped on the train and we were in Paris by 3:30 pm. Really that first day it is hard to enjoy anything anyway you might as well be on a train.
It wasn't ideal but it really was cheaper than anything else I found. Small price differences add up when there are 5 people traveling. I don't recall the exact tube price, because we used the oyster card later that week. Not terribly much.