I'm an accountant and tax adviser as well as a very mediocre bassist. The mileage rate is now 45p per mile for first 10,000 miles. The annual investment allowance (the threshold for writing off assets against tax in the year you buy them, rather than over a number of years) is now £250,000. If you're spending more than that on kit, I really want to come and see you play!!
I'd probably suggest you run it as an informal partnership, so each band member is self-employed. You should strictly register the partnership with HMRC and file a tax return for that each year, plus each partner's individual tax returns. I can't see HMRC getting upset if you instead draw up partnership accounts and then divvy up the profit between each partner, only filing the individuals' returns. As has been mentioned, most bands won't be making any cash after their kit and expenses so I wouldn't bother if you're never making profits. The tax man only gets upset if you're not declaring taxable profits.
Taking it to the other extreme, you can offset losses on your partnership earnings against other income. The losses would have to be pretty big to make it worthwhile though!
I might regret this, but if anyone's got any questions or it's causing them sleepless nights, drop me a pm and I'll help if I can.
Andy