Heathrow to Eurostar

Very, very close.

You need to be at St Pancras an absolute bare minimum 90 minutes ahead of departure- so 1pm.
Whichever route you take into London (tube, HEX, Elizabeth Line, NatEx Coach, even taxi) you need to allow 1 hour- so out of Heathrow no later than 12 noon.

Before that you have to clear the airport- lets assume 1 hour from landing- that is 11am.

That gives very little headroom for the fight being late for whatever reason (or even retimed between now and October), or for any problem in getting into the city, or even congestion on the ground at Heathrow.

You need a good dose of luck IMHO to pull this off.

In an ideal world this wanted to be a late afternoon Eurostar, or a connecting same ticket flight to Paris or a flight into CDG direct (presumably on an open jaw ticket), even an overnight in London.

Cutting it way too close. Your Eurostar tickets should be changeable and probably for no cost since your are outside the 7 day window. Take a look at this link: https://help.eurostar.com/faq/us-en/question/Can-I-change-my-booking.

Thanks folks. We booked on Rail Europe; I'll reach out and enquire about rebooking on the next train. Appreciate your time.

Flight to exit is usually an hour - and an hour will get you to St. Pancras - and then 90 minutes before the train is required. A 1000 arrival gets you onto the 1330 with no margin so you have an extra hour.

So it comes down to how risk averse you are - personally I would be wholly comfortable with wriggle room of an hour, plenty of people would be but many more wouldn't. Although you don't want to rely on it, they will help get you through quicker for Eurostar if they can as the gates are open until 30 minutes before departure. So there is an emergency hour there if required - again, comes down to risk.

But unless you really want to use the train, why not fly as you are arriving at Heathrow?

EDIT-
Original comment deleted as it proved irrelevant.

It's not cost. Having never done it, I am intrigued at the idea of crossing from UK to France by train. We were originally going to spend a night in London ahead of a busy day of train travel; now we decided to break it up and spend the night in Paris instead (hopefully arriving in time, and with enough energy, for some carnard aux pruneaux, and perfect frites, at Le Trumilou).

From Paris we take a noon train to Turin. Two days later we train to Lucca for four days, via Florence. Then Pisa for four days. We will fly back to London for four days before returning home to Pittsburgh. (London is our one European non-stop destination currently. My place of birth, I haven't visited in about 20 years.)

London will look very different to you after 20 years. Too bad you won’t stop.

Safe travels.

We will be there for four days before we return stateside. I am eager to see the change! I hope to visit Churchill's war rooms, under the Mall. Also hoping a friend comes through with a ticket to a Chelsea match at Stamford Bridge. I grew up very near there, though never attended a match (I was ten when we left the UK.) And of course a proper pint or two and a pork pie. :)

I know you have shown your reason (not that you had to) but someone mentioned cost and that the train is less.

Eurostar at the approximate time you want is £97, plus at least £5.60 for the Underground. There is a flight from LHR to CDG that's £77 at 1530 plus a transfer to the city centre for about 6 euros.

Obviously not of interest to you but might be for another reader

Again thank you all. We opted to change tickets and spend the night in London. We'll catch a 7am train to Paris/Nord, giving us over 2 hours to get to Gare de Lyon for our 12:45 departure to Turin. No stranger to easy train travel on the continent, I had obviously neglected to take post-Brexit challenges into account. Now we can have a leisurely ride into the city, drop off the bags, and enjoy that pie and pint for lunch. Thank you all again for your comments.

I am glad that that worked out.

Don't forget to leave plenty of time for check in for that early train to Paris. You need to be sure you meet their requirements.

Have a great trip

A friend living in London has told me he considers 2 hours the minimum lead time for arriving at St Pancras now. He now prefers to fly when he needs to go to Paris. It's quite possible, though, that his concerns are not applicable to such an early Eurostar departure.

As you’re now catching an early train from St Pancras, you can probably get away with less than 90 minutes but it does depend on your attitude to risk. I caught an earlyish train to Brussels the other day - I think it was the third Eurostar train in the day. I joined the queue with I think 85 minutes to go, and got through in 15 minutes. I’m quite risk-adverse and was glad I allowed the margin, but my feeling was that the earlier in the day, the less backlog there is.

I do know that they weren’t letting people join the queue any more than 90 minutes ahead of time, but you might want to give yourself time at St Pancras to buy coffee and food before joining the Eurostar queue, because there are more options “landside” as it were (although having said that, I’m not sure how early the food shops open).

Thanks all, this has been very helpful. We'll go to bed early the night before- having arrived that morning after an overnight flight, even in Economy Plus I'm sure we'll be quite sleep deprived. I'll pop half a sleeping pill, we'll set our alarms, and we'll be sure to arrive St. Pancras with plenty of time. I'll post an update after the trip.

How are you going to get from your hotel to St Pancras at 5am? Be aware that public transportation has reduced service at that time of day and plan accordingly. My son and I had "an adventure" trying to make the 7am EuroStar - it all turned out well at the end, but we did have a few minutes of panic and some wonderful conversations with the street cleaners.

Have a fabulous trip!!

Thank you MapLady, we had considered that and booked a night in a hotel a short walk from the station.

Wonderful - have a great trip!

Update. Now all a moot point. An avalanche has cancelled the Paris to Turin leg of the trip. (This was refunded; the London to Paris portion which preceded it by a few hours may not be. Obviously it's still running.) We will spend the night in London- now near Victoria Station- and fly to Turin the next day ex- Gatwick. Disappointed. However, we may book a late winter trip to London and perhaps we can take advantage of rescheduled Eurostar tickets then.

Oh my! I ‘m glad you were able to come up with an alternative.

Enjoy a pint at the Two Chairman and a stroll via the Cockpit steps into St James Park.

Unfortunate that Mother Nature disrupted your adventure but you are in London so enjoy it.

You probably need to buy the Churchill War Room tickets in advance. The ticket line can he very long. The audioguide for the war room section is really good.

The Imperial War Museum is also excellent. I'm female and have absolutely no interest in weaponry, but I really like that museum. That you can walk right into; It's free/donation requested. The IWM audioguide mostly overlaps the posted explanatory, material, so you can choose whether you prefer to read the info or hear it. There's no need to do both. Tnere's a charge for the audioguide.

Thank you acraven. I too am a fan of the IWM, though I haven't been in many years. We just ordered a scan of my grandmother's hand-written memoirs of WW1 from them. She was in service on a farm in southern England at the time, 16 years old, and the war was several months old before she knew it was happening. She found out when a shell (from where we know not) flew over her head and exploded in a field a couple of hundred yards away. Within a year or two she was working in a munitions plant, building bombs and shells.