Los Angeles to Nice via London

You should allocate at least two hours through London, as the transfer can take longer. However, if you miss your connection and you are booked on a single ticket, you should have no problem catching a later flight. Nice is very popular with the Brits and there are a lot of flights between May and October.

There are typically no non-stops from the west coast of the US directly to Nice. However, depending on your dates, there are like 3 special flights that Air France is running in May from LAX (for the Cannes Film Festival). You may have seen these already. I connect all the time, usually going through a European capital/portal (Paris, London, Munich, Frankfort, Amsterdam, Brussels) or via a US east coast set (Atlanta, Newark, JFK, Philadelphia)

If you buy a ticket for LAX to NCE, the airline will know how much time to allow between flights at the intermediate stop, be it London or somewhere else, depending on which airline you choose

For best pricing you probably should look at multi-city ( aka 'open jaw') flights, i.e., LAX-NCE, LHR-:LAX as a single purchase.

Buy a separate ticket for the NCE-LON segment.

An alternative would be to fly from LAX into Philadelphia on American. They start flying Philly to Nice on May 6th, 2014 nonstop.

I think changing planes at Heathrow is a stressor to avoid.

This year I had three connecting flights through Heathrow with connecting times between 2:15 and 4:00 and there was plenty of time. With that said, I would prefer at least 2 hours.

I'd avoid LHR and CDG if I could, but you may not have other good options. In 2017 I began my trip in Nice. I flew KLM (ticket purchased through Delta) and changed planes in Amsterdam. That worked fine.

I hope you are not landing in Nice on May 1. It's a major holiday there, and there was no public transportation running back in 2017. There were also very few taxis.

In the summer of 2022, I flew SEA-->LHR-->DUB and LHR-->SEA. I used Google Flights to explore a multi-city itinerary because I wasn't doing a typical round trip.

I discovered that the fastest time from Seattle to Dublin had a VERY short time to get from my arrival gate in London to the departure gate for Dublin. I knew I couldn't move that fast after the overnight flight and I didn’t want to deal with the hassle of having to be put on a later flight even though that would be the case with the flights on one ticket.

Farther down on the list of options was a Seattle to London to Dublin combo that had more time in-between, so I chose it. I got the ticket(s) through American Airlines, but the flights were on British Airways and Aer Lingus. I had plenty of time to make the Dublin flight, especially since it was delayed.

I did not pay to select a seat on BA, but rather waited till the free seats were released. That's much easier for a solo traveler like me than it is for a couple. I got a great aisle seat just like I wanted. I did the same thing for the flight back to Seattle with the same results.

It sounds like in your case you may be doing a round trip or maybe spending time in London before flying home. I recommend using Google Flights and multi-city to research the options. I much prefer flying from the US to Amsterdam and on to my final destination, but go through London if that’s the cheapest or most efficient routing. I avoid CDG like the plague. In any case, I allow at least 2 hours between arrival and departure.

My most complicated trip was in the summer of 2019: roundtrip nonstop to Amsterdam on the same ticket with 3 intra-European flights varying days apart (Amsterdam to Lisbon, Porto to Barcelona and Sevilla to Amsterdam) on separate tickets in-between. I found all of them using Google Flights and got the tickets directly from the airlines.

I hope you find some options that will work out well for you both to and back from Nice.

I flew from SEA to Nice via LHR last month, on British Airways, with no difficulty. The inbound and outbound flights were both at Terminal 5. I think two hours would have been enough time, given that everything was on time, but I had a couple more hours than that. Chances are that you'd also use T5 coming and going, but if you need to change terminals you'd want to add an hour or so to your layover. You'll go through security to enter the T5 departure area, whether your flight arrived there or at another terminal. Here's a lot of info about LHR, including how to do transfers. It's not a popular airport on this forum, but I've never had a problem there. (So far....)