In Magistrates' courts, defence appeals to the Crown can be made against conviction (if they've pleaded not guilty) or against verdict/conviction, sentence, or both
When appealing against conviction, the Crown court can confirm, vary, or acquit; when appealing against sentence, they can confirm, increase (up to magistrates max), or decrease the sentence
From the King's Bench Division, both parties can further appeal to the Supreme Court, but it must be on a point of law of general public importance, with permission (leave) granted by the Supreme Court or KBD, although this is rarely used
The Court of Appeal can either disallow or allow the appeal, and based on the decision, they can quash the conviction, order a retrial, decrease the sentence, or vary the conviction to a lesser offence, but they CANNOT increase the sentence
Prosecution can appeal to the Court of Appeal to quash an acquittal on the grounds of new compelling evidence, leading to a retrial being ordered (Criminal Justice Act 2003)
Both defence and prosecution have the right to appeal from the Crown Court or Court of Appeal to the Supreme Court, but it must involve a point of law of public importance, and leave/permission must be given to appeal from the Court of Appeal or Supreme Court