Welcome to Thailand-UK.
How do you know your previous GP has diagnosed you as schizophrenic? If schizophrenia is suspected, your GP would normally have referred you to the local community mental health team where a mental health specialist (usually a psychiatrist) would carry out a full mental health assessment.
GPs would always make a note of what s/he suspects might be the issue, and then refer the patient to a specialist for a formal diagnosis. As your medical record is only viewed by medical personnel who are actually treating you, I’m not sure what legally can be done about the entry in the notes. You could ask your new GP to refer you to the community mental health team so they can assess you and either confirm the diagnosis or correct it.
Are you able to explain why you feel discriminated, insulted, and humiliated?