I booked a return journey with British Airways.
If I miss the first flight can I still use the return flight?
Unfortunately your ticket will no longer be valid if you have missed the first flight. The customer service website of official British Airways confirms the same:
Other Travel Policies:
Your ticket is valid only for the transportation shown on it from the place of departure through an agreed stopping point to the final destination and the fare you have paid is based on that routing. We will not honor that ticket and it will no longer be valid if you do not use the coupons in the sequence provided. Therefore if you need to make a change to the itinerary you must contact us beforehand to determine how this may affect the ticket and remaining travel plans.
Also it is clearly mention if you need to make a change you should contact BA beforehand otherwise with the BA policy, it would be too late.
Technically, the answer to the question of can you use the return after missing the outbound is yes. BUT...it depends entirely on exactly how you miss the outbound and exactly what you communicate to BA.
If you are a simple no-show, the entire itinerary will be cancelled, including the return.
If you contact BA before hand, meaning before the flight departs in most cases, and can demonstrate a legitimate reason for missing the outbound, they might reticket it to only the return. A legitimate example would be a delayed cruise ship.
If you contact BA beforehand and just want to change it, they can reticket it to just the return, but you will be responsible for any fare difference between the one and two way trip. If the one way fare is less, you would even get a refund.