This will be our first trip to Europe and are planning a 1 day trip via Eurostar from London to/from Paris on a Sunday. Plan on taking the first train in the morning to Paris and last train back to London. It appears the first train leaves St Pancras at 8:19 am. I would like to be at the station by 7:15am. I would like to know how long a taxi will take to get us from Heathrow terminal 4 Hilton hotel to St Pancras so I can tell the taxi when to pick us up at the hotel. We are celebrating our 50th wedding anniversary and will be on a 2 week cruise the prior 2 weeks returning the day before our planned day trip to Paris. Do I need to purchase the Eurostar tickets in weeks advance ?
If you go by taxi, I would suggest at pickup time around 6:00 to be safe. It probably won't take more than 45 minutes but the extra time is a safety net. You need to be at the Eurostar no less than 45 minutes before departure for check in, security and French/Schengen immigration. If you get there really early just grab a cup of coffee.
Let me just throw this at you. The Piccadilly Line leaves Terminal 4 and goes to St. Pancras in about an hour's time. It will cost a lot less. No change of trains.
You can save money purchasing your Eurostar tickets in advance and make sure that the train you want to take is not sold out. And get seats next to each other.
Eurostar tickets go on sale 6 months in advance and go up rapidly as your travel date approaches. Traffic would likely be light on Sunday morning but I would suggest you count on one hour or more to get to St Pancras, just to allow for any sort of delay, road repairs, detour, etc. There is no harm in arriving early and you will likely find a place to sit down if you arrive early and give you a chance to get coffee or food to consume on the train. You will have to fill out an immigration card BEFORE you go through French immigration, which is upstairs from the waiting area, and those forms are on the tables in the common area upstairs. Bring your own pen since there are seldom pens available.
billkillean, the first train out on a Sunday is 08:19, it arrives in Paris at 11:47 (2h30 travel + 1 hour time difference), so you have lost the morning. But the first train is 06:18 on a Saturday and 05:40 Monday to Friday.
So, I would recommend:
a) Stay in a hotel near St Pancras (within walking distance). If you are on a cruise the day before, why are you staying in an hotel all the way out at Heathrow?
b) Do your day trip to Paris on a different day, so you can start earlier.
Definately book your Eurostar tickets ASAP (today). Price range is £58 - £338 return!, the cheap seats sell out early, if you buy a ticket on the day, you pay "businessmen's" fares.
The current timetable (up to 27th May) is here: http://www.eurostar.com/sites/default/files/pdf/timetable/UKEN%20Timetable.pdf
We took taxis and Ubers around London last week and had good experiences with both. Ubers were about half price.
Of course you could always just jump on the Heathrow Express to Paddington and the trip is 15 minutes. From Paddington take the tube all the way to St Pancras in about 18 minutes. Since you're just going for the day you won't have any luggage to deal with, right? It's really a simple journey.