I believe that if you earn over £1k from it you'll have to declare it. How you declare it is another matter though, you might have to do self-assessment just for the additional income if your main income is dealt with under PAYE. The other question is if you're getting paid in sterling because you're a UK taxpayer, euros because that's where the tour is, or in USD because the artist you're supporting is American, and also where you're getting paid.