Has anyone here come in from Seattle on British Airway and then transferred onto flight to Nice? Do you recall which gate (A, B or C) you come into? And then which gate does the BA flight to Nice leave from. We have only 75 min layover.
You can Google the flights over a few days and it will list the gates used. As an aside, depending on your flight time, our experience with BA from Vancouver to Heathrow is that they are notoriously late, in part because they leave Heathrow late. The Vancouver flight is late evening so more risk for departure delays in general. We always leave 3 hours for our connection time in Heathrow because it is hard to move through at an efficient pace. Having said all of that, if there are later flights to Nice the same day, they would book you for that if your Seattle leg was delayed resulting in you missing your connection.
The Heathrow airport website has a connecting flights section where you can enter your arriving and departing flight numbers for more information
The good news is you won't have to go through immigration or customs at Heathrow although you will have to go through security.
A of now, both of your flights are into and out of Terminal 5.
Most flights from the USA arrive at either Satellite B or C building. They can be assigned different gates each day. The same is true for departing flights. Security is done at the main Terminal 5 building (A) and once through you can check the board to see what gate has been assigned to your departing flight. This is usually done around 50 minutes prior to departure.
If for some reason you miss the connection, the airline will put you on the next "available" flight which may not be the actual next flight.
If there is a significant delay, you will be entitled to food vouchers, hotel vouchers, and financial compensation. This is known as rule EU 261. Become familiar with the rule as the airline won't offer anyting unless you ask. And even then they will try to offer less than you deserve.