I don't think the developers care either: I've emailed numerous times without an answer, and even less-than-favourable reviews on the App Store failed to elicit a response. At least tell us the reason for the UK- hatred if you're not going to fix it?!
https://en.m.wikipedia.org/wiki/Radio_Garden
“unfortunately the restriction must be extended for an indefinite period due to copyright and neighbouring rights related matters that require clarification. ”
Just like TuneIn, they are behaving like a broadcaster and aren't paying license fees, so I guess they are afraid of similar legal action
Approximately 20% of the stations featured on Radio Garden are accessible via HTTP streams, which would become inaccessible if we switched entirely to HTTPS. Given the recent changes in Chrome's behavior towards forcing an HTTPS connection, we are considering moving back to an all-HTTPS setup in the near future.”