"Best" is always subjective.

But the voucher has essentially no value for a European short haul trip, since you are forced to pay the highest Avios/lowest £ rate, which is very poor value. It ends up working out costing you roughly the same to book in Y (with the voucher upgrading you to Y) as it does in J without a voucher.

The value is therefore to be had on long haul trips, where the same highest Avios/lowest £ rule applies, but it's not as bad value; it just requires you to have more Avios in your account.

In theory you'd extract the maximum value of 130k Avios from a SYD J redemption, although you pretty much have to book the second the seats get released at midnight GMT 355 days out to have a chance of getting them. But you can still save many tens of thousands of Avios through redemptions on other routes that are easier to get hold of.