Program leaving Roku for Acorn?

I don’t believe anyone can force you to do anything with respect to watching TV.  However, streaming channel/apps do add and remove stuff from their platforms whenever they feel like it (really based on contracts and how much some show is earning). If stability is what you want, Blu-Rays may be better. 

That said, looking here…

https://www.justwatch.com/us/tv-show/midsomer-murders

you seem to have huge selection of channels where you can stream that show.

In addition to what Strega2 said, they often give you an episode or a few episodes for free just to suck you in.

Use the search function.

Older episodes are available on many platforms. Acorn probably has the rights for the current and upcoming seasons.