If you are selling work then ypou should be registered as a sole trader and submit am annual tax return.This can be done online.If you are still earning a wage then this can result in you getting tax rebates or, if you are amazingly successful selling, having to pay tax. It means that you can claim for anything that relates to the 'work'such as tools, wood and other materials, etc. Be aware that if you do shows, even the church hall varieties tax inspectors can and do turn up at them to check that you are in fact registered.
I keep a running spreadsheet throughout the year of my income and outgoings which is sent in with the online tax return and that suffices or at least has for the past ten years.
I have heard that you don't need to register if your net income is less than £1000 but I would check this if I were you.
Pete