Can't you just get to the airport 2.5hrs before your return flight and use the airport's wifi in the terminal to check-in online via their free app? Or if you have an old Nokia, do as Caiman said, pay a fiver each, check-in in advance and get an extra hour in the pleasant Polish wilderness?
There's your answer. Download the App before you go. Get to the Airport a little earlier, check-in at the Airport before check-in closes. You'll probably be last to check-in and not get seats together..... same with BA and most Airlines these days!
You'll lose an hour in the wilderness, but at least you won't have to endure LHR and the M4 home.
FR are great, if you just follow the procedures!
EZY do the same, they don't charge you, but if you waited until you got to the Airport to check-in chances are you'd be split up.
Otherwise use the money you saved by flying with FR instead of BA and pay the small amount for your seats.
It would be interesting to see how much the difference in price for the flight was
Unnecessary rant as far as I see.