The rules are national and rigorously enforced.
High rate mobility is a 'passport' to a blue badge.
If you don't get HRM, you have to fulfill one of the other criteria. For children this means they can answer 'yes' to one of the following:
"I am over the age of two and registered as blind (severely sight impaired)
I have either a Certificate of Vision Impairment (CVI) or a BD8 form, signed by a consultant ophthalmologist, stating that I am severely sight impaired (blind) and I wish to be registered as severely sight impaired (blind) with my local authority
I receive the Higher Rate of the Mobility Component of the Disability Living Allowance
I am over the age of two and have a permanent and substantial disability which means that I am unable to walk
I am over the age of two and have a permanent and substantial disability which means that I have very considerable difficulty in walking
I am under the age of three and have a medical condition that means I must always be accompanied by bulky medical equipment
I am under the age of three and have a medical condition that means I must always be kept near a vehicle in case I need emergency medical treatment."
When I applied for DD1's badge the criteria was 'permanent...physical disability'. I was turned down. I appealed, saying that her brain malformation was physical, and it was that which caused her symptoms which made it difficult to walk. The appeal was accepted.
"good reason on an ongoing but hopefully sortable problem"
This is possibly a sticking point. Temporary issues don't count, even if the problem may be there for more than a year, say.