What did you say in your complaint to motability?
I agree it looks to be a cost cutting exercise. However and I can’t stress this enough – the installer should have withdrew/stalled on the Ohme changeover, if they thought there was a detriment to safety and or to request further funding to complete the job to the relevant standard, if not already obtained.
The installer is the electrician, who sits above Ohme or their contractor with knowledge of the latest regulations.