Hi Sussexwoman and jeanrtco, Welcome to FlyerTalk!

If you book an airline ticket, you can abandon it at any point in time. This may include at a connecting city on a one way or simply not using the return portion of a round trip ticket.

However, once you miss a flight segment you can't pick up from where you left off.

In other words If you book a return ticket, London to Paris, and you don't make the London to Paris portion, you cannot fly the Paris to London portion.

As for buying a different return ticket for your trip to Europe, you could buy a completely new return ticket to Europe and just not use the one on BA. Or you could just buy a one way from Europe back to wherever you want to go.