DVLA publishes every vehicle's MOT expiry date. Most garages do not use it. They have a box of cards or a column in a spreadsheet, and a quiet week in February when nobody looks at either.
Every car that has been through your workshop has its reg on an invoice. The system takes those regs, checks DVLA once a week, and when an MOT is due in five weeks the customer gets a text or an email with a booking link. Service reminders work off your own records, the mileage and date from the last job sheet.
Three weeks out, a second message if they have not booked. Then it stops. Nobody wants a fourth.
The booking link drops the slot straight into your calendar with the reg on it. A courtesy car can be asked for on the same page.
This is the kind of thing we build. It runs in your own accounts. How many of last year's MOT customers came back this year?