Time for transfer at Heathrow (separate bookings)

I'd fly in the day before. With a transatlantic ticket on the line, I wouldn't be willing to risk a same day-flight. However, I am very risk-averse in situations like this.

I'd either fly the day before or take the Eurostar the day before as well.

If you are going this spring be aware of the rolling strikes on the French rail system and also strikes on Air France.

Pam

I, too, would fly in the night before. I'll miss a cheap flight if I have to, but not a transatlantic flight as the cost to get another flight is just too high for me.

I too would advise getting to London the day before.

However, if you are determined to fly the same day, I suggest you choose an airline with lots of flights, and book one with plenty of time, like arriving at Heathrow around 8 am. If that flight is delayed or cancelled, there should still be ample time if they put you on the next flight. In any case you should be at Heathrow by 10 am. You will likely have to transfer between terminals, and you will go through security screening as well.

I will make it 5-0; get to London the day before.

Without a connecting ticket. You will go through exit immigration in France and immigration and customs in London, and then departure immigration when leaving London to the US. And finally security. To give yourself adequate time for minor schedule problems would plan to be in London no later than 9am -- maybe 9.30. Personally don't think it is the best plan but if all goes well you should be OK. What is the airline out of Paris?

Frank, we are flying British out of Paris and American out of London.

My concern was if you were flying one of the discount airlines, then there would be more risk. It is a schedule I would not do since the penalty if you miss your flight home is huge.

Another vote for day before. Your flight from Paris is also likely to arrive at T5 whilst your departure is likely to be from T3.

We connected through Heathrow with the same airline and it took us almost 2 hours to go through the whole process (Philly to London to Barcelona and then the reverse). We were running on the way back through because we only had a 2 hour layover. Heathrow is the bane of our existence lol but since we usually fly American/BA, it's cheaper to go through that hub.

we are flying British out of Paris and American out of London.

BA and AA are code share partners. If you haven't bought your BA tickets yet between Paris and London Heathrow (LHR), once you do, contact AA and request your trip be put onto 1 itinerary. If you get an agent who says no, call back and speak with another agent. I know Delta will accommodate this request; I would think AA would too. One itinerary would give you some protection in case your BA flight into LHR is late.

With that protection and if your AA flight departs FROM THE SAME TERMINAL AS THE INBOUND BA, if you don't want to spend the night at Heathrow, you could fly into LHR 4 hours in advance to allow for passport control and security. In the same terminal, you shouldn't have to go through immigration. I speak from experience in that I flew from the JFK to GIB via LHR on two different tickets and didn't go through immigration. I didn't have to leave Terminal 3 and I gave myself a 4 hour transfer time cushion.

So just an idea. That 2pm flight gives you flexibility unlike an early morning transatlantic flight which is nice.

Flying from LHR to GIB ( a British Territory) and flying LHR to the USA, may not be an apples to apples comparison, for the procedural protocol.

One World, the alliance that BA and AA belong to, has changed to a policy of treating separate tickets as separate tickets, even if they are on the same airline. They will not tag bags to the final destination either.
This happened sometime in the past couple of years.

Without checked bags you can do an airside transfer but you do need to go through security.

On a single ticket 2 hours would be needed - on two it is up to your risk attitude.

Thanks for all the advice!
We do not have checked bags and can get a flight that puts us there with approx 4 hours of time to spare.

Here is a link at Heathrow where you can input your flight info and get some input:

https://www.heathrow.com/flight-connections.

You will be flying into terminal 5 (if on the 7:25 out of Paris, landing in London at 7:45) and most likely leaving from terminal 3. I would not choose the 10:35 plane out of Paris, for sure, but am inclined to think I would give the earlier connection a try. Input your info in the link above and see what Heathrow thinks.

As Marco says, you can transfer between T5 and T3 airside if you do not have checked bags. You will also need the boarding pass in hand for the second flight. You will still go through a security screening, but do not need to go through immigration. Just follow the purple “flight connection” signs to the proper terminal for your US-bound flight.

We have done this transfer in 45 minutes, gate to gate, but we were in business class so first off the plane.

But you are still counting on your Paris to Heathrow flight being pretty close to on time. Which is likely, but not guaranteed.

When are you planning to do this? Height of the tourist season or off-peak? We've done this coming into LHR flying business class and had plenty of time to make a connection to Ireland in May. If you get your boarding pass for your long-haul flight ahead of time, you can stay with the connection procedure at LHR, although that still involve several steps. And, as long as you can stay with carryon luggage. Make sure those bags fit the flight requirements for your flight from France ( and make sure this flight is operated by BA and not a partner with stricter requirements). Even though we booked all our flights through BA, the next (separate tickets) flight was on a partner and they wouldn't transfer luggage.

We have done this transfer in 45 minutes, gate to gate, but we were in business class so first off the plane.

Not only off the plane 1st, but have special 'express lanes' to use for being biz class or better.

I would be happier taking a late flight out the night before and spend the night at an LHR hotel, rather than spend the day before stressed with "I hope all goes well in the morning" and have to get up ghastly early to get to CDG for a really early flight.

Happy travels

Yes, the Sofitel in Heathrow 5 is pretty nice. They take Avios, maybe other points, too.

Flying from LHR to GIB ( a British Territory) and flying LHR to the
USA, may not be an apples to apples comparison, for the procedural
protocol.

That's why I brought it up. I flew two separate airlines that did NOT have a codeshare (Delta and BA). I was very concerned about missing the 2nd flight due to a late LHR arrival so I gave myself a 4 hour cushion. As with the OP, Heathrow was a stopover for me. Even though I was on different airlines on separate itineraries, I never went through immigration; I stayed airside as I didn't have to change terminals. So no time wasted on immigration lines at Heathrow. I am hoping that the OP's arrival from Paris and departure to the US are in the same terminal.