A Self Assessment Tax Return is a system used by HM Revenue and Customs (HMRC) to collect Income Tax from individuals and businesses whose income isn't taxed at source. You need to file one if you are:
Self-employed or a sole trader
A company director
Receiving rental income from property
Earning income from overseas sources
A high-income earner over £100,000 per annum
Receiving income from savings, investments, or dividends exceeding certain thresholds
Making capital gains over the tax-free allowance
A partner in a business partnership