Unexpected additional charge for Dallas on Apple TV in the UK

Apple TV is a free application on Apple devices that aggregates video watching into one central location.

Apple TV can also be Apple’s hardware device for streaming video.

AppleTV+ is Apple’s subscription streaming service for Apple Original content.

Dallas is not an Apple Original show and is not available as part of an AppleTV+ subscription.

appletv+ is a streaming thing apple has which only feature apple produced material

it does not cover any of the third-party media they host for purchase or rental

you can read more about it here Apple TV+ - Apple

if you purchased something then you can try to get them to refund you

Request a refund for apps or content that you bought from Apple - Apple Support

For the free movies/shows included in your Apple Music subscription, make sure you have the Apple TV+ tab selected in the app.

If you are using the Home tab, then all content is shown where some can be purchased and others will need a subscription for a different service to watch. The shows you see there are not specifically Apple shows. For example if you have a Paramount+ subscription also, you can watch the content they provide in the Apple TV app.