In the UK, the legal basis for an appeal isn't just being unhappy with a decision; you need
permission from a judge, who grants it if the case has a "real prospect of success" or there's a "compelling reason" to hear it. Grounds for appeal must show the lower court made a serious error, such as a mistake of law, a serious procedural irregularity (like bias or flawed directions to a jury in criminal cases), or that the finding of fact was fundamentally wrong, not just that you disagree with it. Appeals aim to correct errors, ensuring justice, but are limited to specific legal grounds, not a full re-hearing.