I do hope your plans change again and that you can make your original booking.

I can't see that BA would accommodate you otherwise - why would they? You chose not to pay the price of keeping the return sector by changing your original booking.

I think there is a very faint chance that your booking might not be cancelled, but even as someone who often takes a few risks when flying (I don't position overnight for ex-EU as many do, for example) this one would be way too risky for me. I know there have been some isolated accounts of people managing to travel on an itinerary after missing the first leg, but if you exclude irrops where the airline is at fault, then I think the chances are very very slight.