Am I overly optimistic to think we can take an early Eurostar from Paris to London and catch a 6:30pm flight from Heathrow the same day?
It's absolutely doable. If you take the first train at 5:30am it arrives around 9am. This would give you plenty of time to do some sightseeing as well.
The 5h30 train maybe a little ambitious... the first Eurostar train departs Gare Nord at 7h13 rather than the 5h30 time...metros aren't even open until then and the French like to sleep in a bit.
With the loss of 1 hour with the time difference, and the new high-speed rails on the British side you will arrive in London at 8h30. Still a significant amount of time to do things but not sure how many visitors items will be open then.
I recommend the French way - sleep in a little take the 9h13 train arrive at 10h36.
If you will be traveling during the week in particular you will want to book this train as early as possible - this is often a quickly booked train.
You can store your luggage at St. Pancras, the fee is around 7 GBP per bag as I recall for the full day. St. Pancras/Kings Cross station is on the line you will take to Heathrow. (Picadilly)
If I were doing this, the latest Eurostar I would take is the one departing Paris at 07:43 and arriving in London at 08:59. I would store my luggage at St. Pancras and buy a one-day, zone 1-6 off-peak travelcard (travel after 09:30 on weekdays) for £7.00. After exploring a bit of London, I would take the Tube from St. Pancras to Heathrow, leaving around 14:30 to be at Heathrow by 15:30, giving me three hours to check in for a flight back to the US (assuming that's what you'll be doing).
Thanks for the tips. It's good to know that last day won't be just a blur running to the airport. We will have already stayed in London three nights at the beginning of the trip. Don't think we'll have too much sightseeing to do but will have time for a couple of places if we do.
Tim, I was thinking of giving ourselves three hours at the airport. Two years ago, I had to go through two security checkpoints at the Frankfurt airport where everyone had to empty pockets, take out laptop from bag, etc for xray or wand check. Don't know what to expect at Heathrow.
Thanks again everyone.