I can answer “does it matter”: yes, different countries license the wifi frequencies differently, so if the setting is “wrong” you can technically be breaking the law.
I’ve never set this on my Sonos gear mind you, but I do set this on my wifi routers, and they also have an “outdoor” setting to further restrict the frequencies if your antenna is outside, used to conform to local laws.
I have no idea if the UK and US frequencies are different and I’m too lazy to look it up on wikipedia.
Answers to your questions in the link below: