Essentially, it's anything you spend money on throughout the tax year that directly relates to the running of your 'business' — i.e. self-employed performer life. If something has a personal element too (like your phone), you just claim a reasonable percentage.
If you'd feel comfortable defending the expense claim to HMRC as something you genuinely needed to run your performing career — you're probably good.
Quick and simple for on-the-go recording. Here's the whole performance, start to finish:



In a nutshell: whenever you're spending money on something that is genuinely part of running your business, chances are it's deductible. Here are the ones particularly relevant to performers.
Telephone bills. Calling your agent and booking auditions? Line rental and call costs are absolutely deductible — winner! Personal calls aren't, so think honestly about what percentage of use is business and claim that. A third, perhaps? Then 33% of your bill per month.
Working from home. Show research, audition prep, at-home admin — use HMRC's standard flat rates, no need to itemise the electricity bill:
Work from home 101+ hours a month for a year? That's £312 of deductible expenses. Not bad!
Vehicles. You can work out a percentage of actual running costs if you fancy — but the easiest method is HMRC's simplified flat rate for mileage. (Once you use flat rates for a vehicle, stick with them for as long as you use that vehicle for business.)
Example: a UK tour with 11,000 business miles = 10,000 × 45p (£4,500) + 1,000 × 25p (£250) = £4,750 to claim.