Their reply:

The reason that we ask for ID is to safeguard your personal data by verifying that the request is genuine before proceeding with deleting your personal data. This process is consistent with guidance published by the Information Commissioner’s Office. (https://ico.org.uk/your-data-matters/your-right-to-get-your-data-deleted/)

The purpose of this process is to prevent someone unauthorised from requesting deletion of your data, for example where there are shared email addresses, or someone has access to your account or email address, or where someone is spoofing your email address. Please see our Privacy Policy (xxxxxx) for more information about personal data we collect store and process.

Please be assured that when you send ID to our dedicated ID email address, this is automatically and permanently deleted from our systems within 7 days. We do not continue to store or process your ID beyond this time or use it for any other purpose other than to verify your identity to action your erasure request.

If you would prefer not to send ID via email, you can post copies to our address and upon receipt from our team we will then securely dispose of the copies. Please send these to:
Data Protection Team,
xxxx
xxx
xx

I hope the above explains our rationale and allays any concerns you may have. If you have any further questions please do not hesitate to ask.