Response received from Information Commissioner's Office this morning:
Dear <helvella>,
Thank you for your email of 30 January 2020.
Data protection legislation does not specify how to make a valid request. Therefore, an individual can make a subject access request verbally or in writing.
Organisations can ask an individual to complete a form, but the individual is under no obligation to do so. A subject access request is valid if it is submitted by any means, so they will still need to comply with any request they receive verbally, in a letter or a standard email.
Although you can make a request verbally we do recommend doing so in writing if possible, because this gives you a record of your request.
There is further information about how to make a subject access request on our website, via the previous link.
I hope this information is helpful to you. If you would like to discuss this further, please contact me on my direct number <redacted phone number>. If you need advice on a new issue you can contact us via our Helpline on 0303 123 1113 or through our live chat service. In addition, more information about the Information Commissioner’s Office and the legislation we oversee is available on our website ico.org.uk.
Yours sincerely
<redacted name>
Case Officer
Information Commissioner's Office
I hope this clarifies - but I have asked for further clarification about the "signed consent form" that is also being demanded.