I've recently graduated with a masters in Clinical Psychology in Belgium. Is this diploma sufficient to be an accredited psychologist in the UK or would I need additional training?
The appropriate regulatory body is HCPC.
The following titles are protected, meaning you cannot describe yourself as any of the following (professionally in the UK) unless you are on their register:
• Practitioner psychologist
• Registered psychologist
• Clinical psychologist
• Counselling psychologist
• Educational psychologist
• Forensic psychologist
• Health psychologist
• Occupational psychologist
• Sport and exercise psychologist
Information on applying for registration can be found from HCPC here. It is likely that you can apply under "International - EEA mutual recognition rights" - if your course is one that would allow you to practice as a registered psychologist in Belgium and you hold an appropriate citizenship (EEA member state or Swiss).