Voluntary Downgrade UC

We travelled back from Montego Bay yesterday and were asked at check in if we would voluntarily downgrade to PE due to a couple of faulty seats in UC. After a little consideration and a lovely offer of free return Upper Class flights to anywhere on the virgin network we decided why not! Never thought we would get to fly UC again so thought we would jump on the chance with the sacrifice of not getting a bed on overnight flight ( FYI we only paid £900 return for UC so didn’t really feel like we were losing out on too much!) We ended up sitting front row in the bubble so had quite a comfortable flight back.
My question is has anyone had this experience before and what was the process like booking your free flights? I see on our voucher it says G which is rewards so I’m assuming we can only fly on select flights where these are available?
We’d love a trip to San Francisco in the new year so hoping we could use it on that but having a look at the dates we want to go there are no fares available for miles.
Any insight would be appreciated!
My question is has anyone had this experience before and what was the process like booking your free flights? I see on our voucher it says G which is rewards so I’m assuming we can only fly on select flights where these are available?
We’d love a trip to San Francisco in the new year so hoping we could use it on that but having a look at the dates we want to go there are no fares available for miles.
Any insight would be appreciated!