I think you can but it will count as a volountary change so you'll have to pay the £35 fee. Although having said that I haven't done it personally, only the T-355 then add return (for which there is no fee).
The trouble with booking a one way when the outbound is in date (even if not available) is that if it *was* available then you could have done it yourself not on the phone. I have had this on other multi-leg bookings where some of the flights were not available when I booked, so I locked in some of them while waiting for the others to become available. Got charged the fee each time.