How does the price of LHR-ORD-LHR booked through AA (either on AA metal or on BA metal with an AA flight code) compare to the price of booking with BA? If the prices are comparable, and the OP can get a travel agent to book the separate PRG-LHR one-way and the LHR-ORD-LHR roundtrip tickets on a single PNR, then AA would protect in the event of IRROPS on the outbound; might even be able to through-check baggage from PRG.

And if the OP will not have more than one piece of checked baggage, I might even consider flying the PRG-LHR leg in Economy if I could snag an exit-row seat in Economy, rather than dealing with BA's 30" seat pitch in Club Europe!