My T keeps my journals to her (and I presume writings other clients have given her) and her personal client notes in a locked file cabinet. The hospital she works in uses an Electronic Health Record where she keys in her official notes.

I live in Asia. Every country has their own medical confidentiality laws, which may have differences from USA HIPAA. OP is in the UK.

I would suggest talking with your therapist if you're concerned about what goes into your chart / medical record. In my country, different therapy places can have wildly different policies over what goes into the record.