Prepare a short written description of the assistance needed, the appointment reference and date, and contact details for whoever will accompany the person. Any supporting letter from a treating clinician should describe the practical need rather than set out the full medical history.
Send a request the office can act on
Offices act on concrete requests: a wheelchair-accessible route, an interpreter in a named language, extra time, or permission for an attendant to enter. State the need, the date and the reference number in one message, and keep a copy of what you sent and any reply. If the office confirms nothing, follow up before the date rather than assuming an arrangement will be in place.
One message, one page, one reference number is the shape that works. Open with the appointment reference and date, state the assistance in a single sentence per item, name the person who will accompany the applicant, and give a telephone number that will be answered on the day. A clinician's letter, where one is used, is stronger when it describes function rather than diagnosis: what the person cannot do unaided, and how long they can wait or stand.
Hypothetical example: an entomologist submits a detailed medical summary and no statement of the practical need, and the office replies asking what assistance is actually being requested, which costs a week. Keep the sent message, the reply, and any reference the office assigns to the request in one place, separate from the citizenship, employment and credential documents, which are read for a different purpose entirely.