If you are thinking of buying a movie to keep, then there are many services out there to consider, where Sky Store is the main one that i use where i can watch my Sky Store library via my NowTV & Roku players.
Adding to what @schnapps said, I use the JustWatch app.
This covers some 33 sources of movies and TV programmes, including NowTV and the Sky Store, and tells you, for any title you put in, who has it, for what price, rental or buy, SD or HD as available.
It is particularly useful if you use other subscription services; after all, you may not want to buy a movie if you find you can just watch it on an existing Netflix or Amazon Prime subscription that you might have.
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