It looks like this will work if you have a suitable TV with access to NOW:
https://www.nowtv.com/gb/checkout/summary
I'm guessing it will also work on a desktop or laptop. It looks like you buy it in advance, then activate it when you need it.
Has anyone had experience of using it?
PS Sorry MH, I posted this without seeing your reply to Cashy. It would still be useful to hear of others' experiences with it, though.