There are two parts to the Sky Store, renting and purchasing via Buy and Keep
If the film is on Buy and Keep (I haven't checked) then as the name says you pay once and get to keep it.
You sound like you have been rented it instead which means you have a certain time to start watching the film before it is removed.
The Sky Store FAQ about this can be found here:
https://www.skystore.com/n/hgst/help/faq/getting-started
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