When you fill out the medical questionnaire, you also consent to the service gaining access to your medical records. The medical appointment letter advises individuals to bring copies of all relevant medical notes directly relating to those issues declared. If you don't, expect lengthy delays.
Even if not registered at a GP surgery, any hospital visits or call-ins to NHS drop-in centres will be recorded on your medical history held by your current GP. The service is not particularly worried about periods in which a person is not registered with a GP as any relevant history still goes on record.