Can the UK prevent you from leaving if you did not pay your taxes?

Yes, the UK can potentially prevent you from leaving the country if you have unpaid taxes. HM Revenue and Customs (HMRC) may issue a "notice of departure" that can lead to restrictions on your ability to leave. This is primarily enforced for significant tax debts, and the individual may be required to settle their tax obligations before being allowed to travel. It's advisable to resolve any tax issues to avoid complications when planning to leave the UK.