The Roku app is not a substitute for a Roku device, but perhaps someone from the UK can give you better information. In the US there's a multitude of "channels" as Roku calls them (most people would call them apps) that are available on a Roku device that the app doesn't give you access to. You may already have access to some of them through Sky Q like Netflix, Disney+, etc., but there's a lot of niche "channels" that may not appear anywhere else.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.