fly cdg to lhr

Yes you will almost certainly need to collect any checked luggage from an Air France flight which will also require going through immigration, doing the transit landside, by bus or rail between terminals, rechecking the bag, then going through security.

The only exception would be if you have this journey booked on a single ticket which is unlikely.

British Airways would allow through checking if you had a Paris-London flight from them even on a separate ticket.

I'd take the Eurostar Paris to London and then out to Heathrow via underground or Heathrow express or other. To me that's a lot less time and hassle than going out to CDG (unless you are staying near there), going through airport security, etc etc.

Have you priced the non-stop from Paris to Seatac? It saves a lot of time...I'll usually pay more for a nonstop just because it's a less exhausting way to travel when I can.

It sounds like you are considering booking two separate tickets.

If you do this, I suggest staying overnight in London or out at Heathrow before your onward flight.

Here is the long explanation as why you should have a day between flights booked on seperate tickets. Especially an expensive trans-atlantic flight: https://www.tripadvisor.com/Travel-g41707-c207311/Newton:Massachusetts:Connecting.On.Separate.Tickets.html

As someone else suggested,you should take the Eurostar from Paris to London and then on to Heathrow. The way things are in airports these days with security and all,the train is a better way.