It did not use to be 100% available, Roku denied for a long time ship their Roku Streaming Stick 3600 to the UK. So you had to find a person that had bought it in the US, or trick them into sending it to you.

When I asked support about it they said "The 3600R is not supported in the UK, so I'm afraid it will not work", however it is now selling on Amazon.

Some believed that it was because of Sky having a deal with Roku (Sky has invested in Roku), and because of that they wanted to brand their own Rokus, or something like that.

I am glad that they have since come to their senses and agreed to sell Rokus in the UK.