Generally, there is no right of appeal for visit visa applications. However, that does not mean that ECO's have no accountability. You still have two options:
- Ask the ECM for a review. If there is an obvious error made by the ECO (such as reading the numbers on the application form wrong), the ECM will be able to overturn the ECO's decision. My friend was successfully able to do this. In the refusal notice, the ECO had referred to parts of the application which were materially different from what was actually stated.
- Judicial review. If you believe there has been an error of law, such as not following the proper procedure, you could be able to get the decision quashed through a judicial review. However, you would need a solicitor and be prepared to bear the expenses. Normally, a second application would be a better choice.
Depending on why you think the decision was wrong, you should also contact the British Embassy/High Commission in your country, the decision making hub, the independent inspector for the UKVI, and any other relevant bodies that you think could make a difference.