The issue then is that the law seems quite daft. These roadside bays are almost always placed at the request of the resident and therefore a law which prevents a common sense approach and exacerbates parking issues in crowded streets seems counter productive.
It makes perfect sense to have regular public parking disabled bays a universal size - after all, you never know when somebody with complex access requirements may need to use a bay outside a shop or in a public carpark - but it doesn't make sense for these bays.
My aforementioned grandmother drives a Matiz - it is tiny. Her road is congested - a 6.6 metre long parking bay outside her house would be enormously excessive, cause animosity amongst neighbours and deliver no credible benefit to her or anyone else. Fortunately, her disabled parking space is a regular size!