Why isn't there an option to buy multi-month entertainment (or cinema) passes directly from the NOW TV site? At the moment they have to be bought from a retailer, and there is sometimes the issue of codes not being activated by the seller properly. I had to have a second 5 month entertainment pass sent from Amazon before it worked. Cutting out the middle-man would make life easier, and I assume make more financial sense to Sky.
In the meantime you might find either Game or Paypal Digital Gifts more reliable - they email the codes to you after purchase so it's less hassle than the physical gift cards.
Also worth rather than automatically applying your next voucher, try cancelling the pass first. When you go to cancel your pass choosing the option that it costs too much and see if they offer you a deal that works out cheaper than the gift cards. If you don't get offered anything better just apply another voucher instead.
@Andy Thanks for the information, I didn’t know some retailers could email a code. That would make things a lot easier. I do cancel before applying a voucher, but haven’t had an offer for a while now. I must have used up my allocation for the time being.