If you’re a self employed musician it’s highly likely you’ll fall under the trading allowance threshold. If you do declare yourself as a musician on insurance, you can claim back a proportion of that as an expense.
Personally, I’ve never declared to my car insurer that I play music, in the same way I don’t tell them that im a serious cyclist, love running or am a stone overweight, because it’s nkt relevant.
https://www.litrg.org.uk/latest-news/news/211202-are-you-using-trading-allowance-correctly