Buy and/or Rent movies can be selected from the Store tab/section in the Apple TV app. You may alternatively use in-app Search 🔍, but the search results could sometimes be non-Store titles (instead e.g. subscription-only titles).
As future films are excluded per your description, don’t select Pre-Order titles; one of the categories. That will also be listed for titles that this applies to, in the movie description.
Prices vary by title (that is up to the choices of the supplying studio), so your mentioned £ amount is a bit unclear to me. Please refer to the exact phrasing in Apple’s message. Some titles might be out of that range.
Most titles will be available for either buy or rent. A few titles will be buy-only, and a few titles will be rent-only. That is up to the choices of each of the supplying studios.
Buy movies and TV shows from the Apple TV app - Apple Support
Rent movies from the Apple TV app - Apple Support