The UK Amex 2-4-1 has to depart from the UK (one way or return) so trying to book from XXX back to the UK won't work.

Either set up the return flight search you want online at T-355 and attempt to book the return in one go. This saves change fees but will take 1 more screen to select yhe outbound.

Or book the outbound now and either call to add the return at T-355 or if you have enough Avios book the return one-way seats online and call to have the voucher applied and half the Avios refunded (can someone confirm this still works).

Edited: seems the new voucher doesn't have the UK origin requirement (guess because it can be used on EI and Iberia).