Ignore Brexit. It’s not going to affect you.

but yes it’s possible to transit in this way and remain airside so you only have a passport check and security.not immigration (which you will do in Paris) as part of the transit process.

BUT if you have luggage AA won’t tag it through to Paris in which case you would have to enter the U.K., collect your bags and then check them in with whichever airline you are using to get to Paris on.

Also if your AA flight is late and you miss your Paris flight AA won’t rebook you and nor will the onward airline unless you have a flexible ticket.

there is a very useful planner on the LHR website which explains the process.