PS. I appreciate you are trying to help - FYI I am also an I.T. professional and you can be assured that I have considered my requirements carefully! I am only trying to establish whether the 3910R WAS (I know it is not currently) sold in the UK so that I can determine if they MAY come up on eBay at some point in the future.
Late response - The 3910 does support Miracast-based mirroring, and as of OS10-4195 and RokuCast 1.9.6 it also supports it in Composite output mode (previously it did not: https://support.roku.com/article/208754928)
Additionally as of OS10-4195 (and AirPlay 1.14.104) it supports AirPlay2 (casting & mirroring), in both HDMI and Composite mode.
DIAL and Play-on-Roku based casting also work in both HDMI and Composite mode.
StreamerUser Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.