I bought a Roku streamer last week and everyone tells me that the best channel is Plutotv. This channel is not on my list. I am in the UK.
At the time of writing, Pluto TV is available in the following countries:
Do you not see it in the Roku Channel store?
Or here for the en-gb Channel Store:
Note that if you somehow managed to create a non-UK Roku account it will really mess things up if you really are in the UK.