What pulls you into a UK return from abroad
Non-residents are still taxed on most UK-source income, and Self Assessment is how HMRC collects it. The common triggers are UK rental income (which makes you a non-resident landlord), employment duties physically performed in the UK or a UK directorship, some UK pensions, gains on UK land and property, and any notice to file from HMRC. Your position under the Statutory Residence Test decides what is taxable, not whether a return is due. If none of the triggers apply and HMRC has not asked for a return, you generally do not need to file.
How expats actually file
Almost every non-resident return needs the SA109 residence page, and HMRC's free online service does not support it. That leaves three routes: a paper return, due by 31 October after the tax year ends; HMRC-recognised commercial software; or an agent filing through professional software. The last two keep the later 31 January online deadline, which is also when any tax is due. Late filing starts at a £100 penalty even if nothing is owed. Our expat Self Assessment guide covers registration from overseas, the forms you need and paying HMRC from abroad.
Your new country will usually tax you too
Filing in the UK does not settle things where you live now. Most countries tax their residents on worldwide income, so the same rent or pension can be taxable in both places. The double tax treaty between the UK and your country of residence decides which country taxes what, and credit relief is generally capped at the lower of the two countries' tax on that income. Getting the treaty side right is usually where the real money is, so check your destination's rules alongside the UK filing.