LiviLion,
Cancel and rebook perhaps .... but saying I'm wrong in suggesting you can magically transform a return booking into a one way by dropping a leg is wrong. The ticket was still sold as a return, so it becomes invalid if you don't fly one leg (fundamental fare rules still apply, do they not !).
The T&Cs do state....
Reward tickets are subject to IATA fare construction rules.
They also state:
One way tickets can be issued as Rewards on British Airways flights at 50% of the Avios Points required for a return journey to the same destination. Open tickets cannot be issued as Rewards.
The OP might also want to take note of the change T&Cs.....
14.1. Changes to bookings for Reward travel on flights operated by British Airways or Airline Partners are permitted only in respect of date, time, class of service and (subject to Clause 14.2.) route. Such changes may only be made by the Member or an individual nominated in accordance with Clause 3.13. If more than one Avios & Money price option is available for a British Airways flight, it is not possible to change the price option chosen by a Member after a booking has been made. All permitted changes are subject to capacity limitations and may not be made after the time 24 hours prior to the scheduled departure of the first flight in the Reward travel itinerary. A fee as specified on ba.com may be payable for each permitted change. For the avoidance of doubt, name changes are not permitted.
14.2. The route applicable to an Reward travel booking may be changed provided that the new route selected would, as at the date the change is requested, require the redemption of the same amount of Avios Points as was redeemed in making the original booking. Route changes are only permitted for Reward travel bookings which (other than for the payment of taxes, fees and charges) are purchased using Avios Points only.
14.3. If you make a Non-Flight Reward booking a per product fee as specified on ba.com will be charged in addition to any fees levied by the Partner for each change made. Changes are not permitted inside 72 hours of travelling.