Allow up to one hour to clear immigration/passport control, 15 minutes or so to walk to the Tube station and buy your tickets, and one hour to take the Piccadilly Line to King's Cross/St Pancras. There is a mandatory 30-minute check-in deadline for the Eurostar. I would add another hour for the possibility of a late arrival at LHR. Put it all together, and you'll need a 4-hour connection time. That means that the first train you can safely book is the one departing at 12:24.

Before you buy Eurostar tickets, you might double-check the availability of mileage seats from LHR to CDG on the day of your arrival. They free up seats if demand isn't great. Happened for us -- though on a different route.

As long as you don't have too much luggage, the tube to St. Pancras should be fine. No change is necessary. With any luck, you should be out of the airport before the morning rush hour, which is later in England than in the US. All the alternatives -- taxi/"shuttle"/train + taxi or tube -- are more likely to encounter rush hour traffic problems.

Thank you, TimS! I'd rather do it that way than cool our jetlagged heels at LHR all day.

Oh, thanks Mimar. Good point. I assumed the seats would be gone forever.

By the way, is there a Sunday morning rush hour in London? My city slows way down.

"is there a Sunday morning rush hour in London? "

Inbound on the tube, no. But two days before Xmas the Eurostar will be rammed from 0530 (London is France's 5th or 6th largest city - and probably after Hollande's tax proposals, moving up to 3rd or 4th - and they all go "home" for Xmas), and it's absolutely essential to buy tickets at the earliest possible moment.

There's a different kind of rush hour every Sunday on the roads from around 1130-1430 inbound from the west. But after the last Fri or Sat before Xmas, central London usually ceases to attract much shopping traffic - though with the long runup this year from the last general working day (Dec 21) to Xmas itself, the pattern might be disrupted

Many thanks, flanner. Excellent help.

Is Paris England's 2nd largest then?

PS:

Sorry about the bold. "it's absolutely essential to buy tix ASAP" was the bit that ought to have been emphasised

Flanneruk, very interesting about the French moving to London since Hollande’s election. All the better for the Brits, eh? Especially if they are people of means.

I suppose there is a down side to this scenario also – right?

Not at all, flanner. I need all the emphasis I can get.

As of today you can book Eurostar tickets at eurostar.com up to and including 8 December. I'm guessing that next week you'll be able to book up to and including 15 December. If you check next week and find that I'm right, then you'll have a good idea when booking will open for 23 December.

Oh, yes! TimS. Thank you so much.

My daughter and I will be taking Eurostar back to London early January, while my husband flies home from Paris. It sounds too risky to wait until the return ticket is available. This is a case where I might not feel guilty buying a round trip ticket coming back, intending to use only one way.

Just make sure to build in enough time for late arrivals. We were three hours delayed on the ground as they fixed technical problems on the plane before we took off.

I guess if you miss your train you're just out of luck? Especially when the next one is mobbed with holiday travelers.