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It's an Avios redemption it appears (if it's an AA Miles redemption then this is out of my skill set!).
In which case because you want to dump the first sector it gets a bit sticky. You should be able to cancel the entire trip so long as you are more than 24 hours off departure, and get a refund (there maybe a small fee though). You may even be able to do this online and get everything refunded nearly instantly. However you would then need to be rebook the original return, and returning your first booking to the pool may mean it gets swallowed up, never to be released again. So your starting point is probably to see if there is still redemption space for the return as a single ticket, then to call BAEC, but I don't think it is guaranteed. If your underlying ticket is via UuA it gets even more complicated and less feasible.
If it was in the other direction (keeping outbound, dumping return) it gets easier.