Not sure if UK Wi-Fi channels are much different than US, although I think they may be.
I have all my Echo devices running on the lower 2.4G band, not the 5G band. Why, because they don't need 5G; they are audio devices only, not video devices that basically require 5G for great video streaming. Also, for the Echos, using the 2.4G greater range capability is a plus for Echos. I reserve the 5G band for any device that can stream VIDEO, meaning FireTVs, Chromecast, phones and tablets, and place slower and audio devices that just stream AUDIO on 2.4G.