The client does not know their confirmation statement is due. They know they got a letter from Companies House eight months ago and put it in a drawer. They will find out when the penalty letter arrives, and then they will ring you.
Your side is a spreadsheet, or a calendar with 80 recurring events, or a memory. All three fail the same way: they are only right on the day someone last updated them.
Companies House publishes the accounts due date and the confirmation statement date for every company, free, through its API. VAT quarters follow from the registration. Payroll dates follow from the pay frequency. So the tracker fills itself from the company number and corrects itself when a year end changes.
One view. Every client down the side, every deadline along the top, colour by how close. Click a cell and see what has been received and what is still missing.
Reminders go two ways. The client gets one 30 days out and one at 10, in your words, from your address. You get yours before the client does, so you are never surprised by your own reminder.
This is the kind of thing we build. It sits in Google Sheets or on a small page you can open on your phone, fed from your own data. Which deadline worried you last month?