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?
https://channelstore.roku.com/details/0719d122fc30b9f282fa106a903ab1d6/pluto-tv-its-free-tv
Or here for the en-gb Channel Store:
https://channelstore.roku.com/en-gb/details/aba737c579062421263f5ee30b814174/pluto-tv-its-free-tv
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.