Has anyone ever been able to use this on an ex EU booking. e.g. AMS-LHR-JFK but just upgrading the LHR-JFK leg of the booking? Would be useful for saving on taxes... Terms and conditions don't state booking needs to originate in UK but others here have said that rule exists.
Also, does anyone know if you could use it to book two one ways, for the voucher holder and partner, where the one ways were on different dates e.g. because I want to fly on one Date and partner is to come a few days later. Any likelihood of this being allowed?