For the majority of UK patients, medical records are held by your local GP surgery. For solely private patients, records are similarly ordinarily kept by your private GP. In all instances from previous UK clinics, there's a legal requirement to ensure your medical records are accessible to you. To obtain your 'Summary of Care' record, which ordinarily details your entire past medical history please either: ring your practice, email in (sometimes ignored) or request a physical copy of your records from a receptionist in-person. You may also consider filing a 'Subject Access Request' to obtain a full GDPR account of data held on you as a patient.
A 'Summary of Care' should ideally document your previous diagnoses, operations and overall historic healthcare including previously prescribed medications, and if not, the clinic you're joining may request further evidence of your suitability for medical cannabis before making their own informed decision.
Patients joining the UK from abroad must request an outline of their full medical history including previous and current medications used. For American patients this can typically be obtained from your Primary Care Practitioner. You will have to be separately consulted in the UK for approval, regardless of any previous medical cannabis prescription held in territories such as Australia, Canada and the USA - the same applies for EU and worldwide patients. Medical records from abroad are ordinarily accepted by private UK medical cannabis clinics, though please ensure to include a full documented medical history for consideration.
"I'm travelling short-term, can I access a UK medical cannabis prescription?" In short, no, while you could incur great expense yourself attempting to do so - UK clinics typically aren't willing to accept short-term patients. If you're staying in the UK for 3+ months it's more likely you'd be considered/accepted by a clinic here.