Why don’t you just play the system the other way round: when you’re ready to trigger the voucher buy a £1500 refundable ticket and cancel it after voucher issue, before year-end. Two wrongs and all that...
To be marginally safer I’d do the £1500 before the triggering transaction, rather than as the triggering transaction. Just, IMO, a chance that reversing the triggering transaction is a little more likely to be flagged up than simpy dropping below the threshold.