score:24
Of course it's possible.
Depending on which day of the week you're traveling, the last feasible departure from Paris may be the 13:13, which arrives in London at 14:39, giving you more than two hours to make your way to Heathrow. The Piccadilly line, going directly from Kings Cross St. Pancras to Heathrow, takes about an hour.
(Beware that Eurostar has a complex airline-like check-in process at departure, so for this train you'd need to plan to be at Gare du Nord around noon).
You'll definitely want to book the ticket in advance. There's a chance that you can walk up and get a ticket on the day, but then again there might not be any seats left by that time -- and they don't take standees.
Note that the train may not be your cheapest option. If you want the train ride for its own sake, go for it! But otherwise flying from CDG to Heathrow could easily be competitive on price, even when booked separately.
And especially if you haven't booked your flight from London yet, there's a good chance that you can buy the same flight plus a connecting flight from Paris for very little extra cost, and get the extra benefit of rerouting in case of the connecting flight being delayed.
Upvote:6
This is certainly possible. You will definitely want to book your Eurostar ticket in advance, for two reasons. Firstly, the Eurostar can and does sell out for specific journeys, especially in the summer. Secondly, walk-up tickets for this journey are extremely expensive, around β¬230. Booking even a fortnight in advance could lower this substantially.
Eurostar has a minimum 30-minute check-in which will include passport control. Allow a little extra time if you're not an EU/EEA passport holder. In this instance, also be aware that the UK is not in the Schengen agreement and visa arrangements differ between France and the UK. You will need to clear UK immigration at Paris Nord.
You'll want a clear two hours at Heathrow plus at least 90 minutes for the transfer from St Pancras to Heathrow (by tube or tube-plus-train). This includes a little safety time. If missing your outbound flight would incur significant extra costs, I'd say allow at least four hours between your planned arrival at London St Pancras and your Heathrow departure.
The direct Piccadilly line tube route to Heathrow does not have air-conditioning and is very warm in July and August. The route via Paddington (Circle/Hammersmith and City line plus TfL Rail stopping train) takes a similar amount of time and is far cooler, but costs a bit more.
Your baggage will be security-cleared at Paris Nord but this is not as arduous as flight regulations - for example, you can carry liquids over 100ml on board so a picnic lunch with drinks for the Eurostar is entirely possible.
A connecting flight may well be quicker and cheaper, despite the inconvenience of getting to a Paris airport. It's worth noting that the journey on Eurostar is not visually spectacular and the tunnel is a bit of a non-event, even boring.