I received a companion reward for renewing my gold membership this year. Can I use it to book a companion seat on a non-flexible UC fare or can it only be for the highest price flexible fare?
You need to book a full J class fare to use the companion voucher in UC, so you cannot use on a discounted Z,R or D.
In PE it is S or W fare code, and in Econ it is Y,B,L.
The general consensus is that they work best/give best value for money, when used with an S fare PE ticket, when the difference between that and the lowest PE K fare code isn't that great a lot of the time.