I used my GUF2 voucher to New Orleans this week, coming back from NYC which i couldnt book online. The price the agent quoted me, was vastly different than if i like the OP booked 2 WT+ seats, and then called back to use the voucher. In my case it was 1k GBP difference. Often had this issue with agents where the price is vastly higher and not by 30 gbp.