Get an accountant and his charges can be allowed against tax too. I use John Heydon, (as do many folkies, he's one himself so understands what we do) and he can get your loss (you *will* show a loss) offset against the tax on your day job. That's as long as the revenue accept that you are running a business and not doing it for a hobby. The appearance of accountant's charges helps with
Mileage at 40p/mile (up to some limit, then it's 25p) is likely to be your biggest declarable expense. Everything else you mention, probably, a certain amount for subsistence, and a proportion of your home running costs (heat, electricity etc) because you are using it as an office for business admin. Stationery, postage, magazine subscriptions, materials for promotional literature, phone calls, internet costs... anything relevant to the business.
You can include mileage for trips where you don't do paid gigs, if it can be considered promotional - e.g. travelling to a folk club to do a floor spot with view to getting booked. And the cost of getting in at the door.
You can get tax relief on capital expenditure too, but not all in the year you spend it - effectively it's as if the cost was spread over four years typically. You really need to get advice on how to present that properly to the tax man, and on everything else - don't trust me!