Can occupational health get information from my gp without getting my consent?

Usually not. Occupational health acts as an agent of the employer. They have access to all medical information relating to workplace injuries, regardless of employee consent. Other than that they can only see medical information that you give permission for.
No but... Privacy laws apply to occupational health as much as to any other health care information. Note, however, that if you knowingly lie or withhold information on an occupational health form, not only could you be endangering your own life and health by limiting your employer's ability to properly protect you but you could also possibly be charged with a serious crime.
No. The medical records of your private physician can not be released to anyone without either your consent, or there is a valid court order for the release.