Mainstream music in Britain died about 15 years back. It's all over produced factory pop. Made by an industry style conveyor belt to a standardised format, with a "different" group or person fronting it.
Then there's the mainstream media promoting it's politically correct chosen few. This rings already stale and stagnant music even further.
Until we end up with endless PC factory pop with a female agenda, like adele and beyonce. Or boy bands that sing about being lapdawgs for profit.
There's loads of great bands and styles of music on many platforms.
The mainstream of music, like all mainstreams such as media and news, is a place for the politically correct lowest common denominator bullies to push their ideals.
That's why lots of people are using the tributaries and gives now.